Animal Endocrine Clinic

The Cooking of Commercial Pet Food

2012-03-02T11:57:00.000-05:00

As an endocrinologist, I have now been in practice for a very long time (over 3 decades). But the longer I practice and care for dogs and cats with a variety of hormonal problems (from diabetes to thyroid disease to Cushing's syndrome), the more I realize the vital roles proper nutrition and diet make in the overall success of the patient's treatment.

Susan Thixton has an excellent website — TruthaboutPetFood.com —that I recommend to anyone interested in learning more about pet foods. After reviewing the website, you might want to subscribe to the 'Truth' Newsletter to keep up to date with her many reviews and alerts.

In one of this week's posts (The Cooking of Pet Food), Susan provides a link to an article published on the Pet Food Institute website on "How Pet Food Is Made" written by Greg Alldrich, PhD, a consultant to the pet food industry. In this article, Dr. Alldrich explains that the cooking process of pet foods — both kibble and canned —"provides a number of benefits, including convenience, enhanced flavor and texture, improved consistency, pathogen control and decreased spoilage."

He continues "However, extensive processing can increase variability, destroy essential nutrients and create unwholesome by-products. From a formulator’s perspective, this creates a dilemma regarding how to assure the diet is sufficiently fortified while avoiding excess after accounting for processing effects."

In this article, Dr. Alldrich encourages the pet food industry to learn more about how pet food ingredients are affected during the cooking process.

Links:

TruthAboutPetFood.com website http://www.truthaboutpetfood.com/
Pet Food Institute website www.petfoodinstitute.org
The Cooking of Pet Food www.truthaboutpetfood.com/articles/the-cooking-of-pet-food.html
How Pet Food Is Made www.petfoodinstitute.org/Index.cfm?Page=HowPetFoodisMade

Warning: Serious Illness In Dogs Linked To Chicken Jerky Treats

2012-02-23T23:05:00.000-05:00

Chicken jerky treats imported from China appear to be responsible for hundreds of recent reports of mysterious illnesses that can mimic diabetes mellitus in dogs. This syndrome has results in the death of some deaths of the affected dogs, which has prompted a renewed warning for pet owners by the Food and Drug Administration (FDA).

In 2006, China was responsible for the melamine contamination of the food additives known as wheat gluten and rice protein concentrate. This triggered a nationwide recall of several dog food items from a variety of manufactures. This time the item of concern is chicken jerky treats.

FDA issued a cautionary warning regarding chicken jerky products to consumers in September 2007 and a Preliminary Animal Health Notification in December of 2008. After seeing the number of complaints received drop off during the latter part of 2009 and most of 2010, complaints to the FDA started increasing again in 2011. In November, the FDA announced an official cautionary warning for consumers feeding chicken jerky treats to their dogs (1).

Now amid reports of more than 500 dogs sickened by chicken jerky pet treats imported from China, FDA officials have begun collecting and testing chicken jerky treats upon import, analyzing samples for evidence of dangerous toxins. To date there hasn't been a specific cause or contaminant identified, but the FDA is looking into melamine and diethylene glycol, chemicals used in plastics and resins.

Dog Treats can Cause Fanconi Syndrome and Kidney Failure

Dogs that consume the imported chicken jerky food products routinely can develop a Fanconi-like syndrome (2). This is an uncommon condition which affects the kidneys and causes them to leak glucose (sugar) and other electrolytes into the urine.

Clinical signs or symptoms of Fanconi syndrome in dogs can include decreased appetite, decreased activity, vomiting, and increased water consumption and/or increased urination. Blood tests may indicate kidney failure (increased serum urea nitrogen and creatinine concentrations). Urine tests may indicate Fanconi syndrome (high amounts of glucose in the urine). The increased glucose excretion in the urine (glycosuria) can be confused with diabetes mellitus, but dogs with Fanconi syndrome maintain a normal blood glucose level. If left untreated, severe renal failure can develop, and death is possible.

Until more is known, the FDA has made the following recommendations (1):

Chicken jerky products should not be substituted for a balanced diet and are intended to be fed occasionally in small quantities.
Consumers that feed chicken jerky products to their dogs should monitor for symptoms of decreased energy, decreased appetite, vomiting, diarrhea, increased water consumption, and increased urination.
If a dog develops any of these signs, you should stop feeding the chicken jerky product.
Owners should consult their veterinarian if signs are severe or persist for more than 24 hours.
Veterinarians and consumers alike should report cases of animal illness associated with pet foods to the FDA’s Pet Food Complaint site.

Bottom line

At this time, I would encourage you to only purchase products that are made in the United States. The first thing that I do when looking at a bag of pet treats is turn it over and look for the country of origin. At least for now, I would not buy any treats that are imported from China.

I also encourage pet owners to limit treats to less than 15 percent of the overall calorie intake for their dogs. Please keep in mind that raw or cooked vegetables such as carrots, green beans, and broccoli make wonderful treats, especially for those dogs who are struggling to lose weight.

References:

FDA website. FDA Continues to Caution Dog Owners About Chicken Jerky Products
Hooper AN, Roberts BK. Fanconi syndrome in four non-basenji dogs exposed to chicken jerky treats. Journal of the American Animal Hospital Association 2011;47:178-187.

3 Things You Can Do To Help Your Dog Lose Weight

2012-02-13T22:00:00.001-05:00

We are an overweight society and our pets are no different. Over half of American dogs are overweight, and up to 25% are obese.

Dogs with obesity are more prone to a variety of health problems, just like people. These include an increased incidence of diabetes, asthma, and hip and joint problems.

Losing weight can help overweight dogs become healthier and happier. It can also help them to live longer.

Here are some things you can do tomorrow that can help your dog start losing weight.

Cut the treats in half. If you must feed treats, cut them in half or give half as many in a day. This can help cut calories. Switching to carrots may help.
Exercise your dog more. Ask your vet if your pet is healthy enough to tolerate more exercise. If you play with your dog every day, play a few minutes longer. If you go on a walk, go a little farther. This helps burn calories.
Lower the amount of calories your dog consumes each day by changing your dog's food. I like to use diets lower in carbohydrates and higher in protein.

There are several diets formulated for weight loss. Discuss it with your veterinarian to learn more. But remember, whatever diet is chosen, portion control is a big issue.

Although the most common explanation for an overweight pet is simply a lack of exercise and too much to eat. But what if you feed your dog sensibly, exercise adequately, and your dog still has a weight problem?

There could be a number of underlying diseases may be causing your dog to become overweight or obese. Hormonal diseases such as hypothyroidism and Cushing’s syndrome commonly cause weight gain. Steroid pills or tablets with cortisone-like drugs could also be contributing to the obesity.

If these 3 simple hints don't help, ask your veterinarian if an underlying disease could be part of the problem.

Obesity Epidemic Worsening U.S. Pets

2012-02-10T13:28:00.000-05:00

A recent survey by the Association for Pet Obesity Prevention (APOP) found that 53% of dogs and 55% of cats in the U.S. are overweight or obese. In many of these dogs or cats, owners are unaware of their pet's weight problem, a phenomenon dubbed the "fat pet gap" by veterinarian Ernie Ward, APOP's founder.

The study finds that more pets are becoming obese and the already overweight pets are still gaining weight.

Click on the link to see the full CNN article (Obesity epidemic strikes U.S. pets) published on February 4, 2012 by Josh Levs.

Click here to learn more about the Association for Pet Obesity Prevention (APOP).

A Day in the Life of Penny, a Hyperthyroid Cat Being Treated at Hypurrcat

2012-02-08T08:41:00.001-05:00

Penny was treated this week with radioiodine (I-131) in order to destroy her thyroid tumor and cure her hyperthyroidism.

As you can see, she is enjoying her vacation at the Hypurrcat spa!

For more information, please visit our website and check out our Hypurrcat Facebook page.

Dr. Peterson appointed to Executive Board of the Association for Pet Obesity Prevention

2012-02-06T12:55:00.003-05:00

Dr. Peterson was recently appointed as the newest Executive Board member of the Association for Pet Obesity Prevention. This new appointment comes on the heels of Dr. Peterson's recent in-depth analysis of small animal nutrition and the role the pet food industry plays in what we feed our dogs and cats.

About the Association for Pet Obesity Prevention (APOP)

The Association for Pet Obesity Prevention (APOP) has launched campaigns to fight pet obesity within the veterinary medical community, veterinary schools, and state and local veterinary organizations, and has reached out to various media outlets. We are made up of dedicated veterinarians and veterinary healthcare personnel who are committed to making the lives of dogs, cats, all other animals and people healthier and more vital.

APOP is not affiliated with any veterinary industry corporation or business in any manner. This neutrality is critical to our ability to provide unbiased information and advice to veterinary healthcare providers and the pet-loving public.

APOP was founded in 2005 by veterinarian Dr. Ernie Ward, a competitive Ironman triathlete, certified personal trainer, and accredited USA Triathlon coach. A key component of APOP’s mission is to develop and promote parallel weight loss programs designed to help pet owners lose weight alongside their pets.

To see a list of the other APOP Executive Board members, click here.

Avoid a Pet Emergency: Super Sunday Parties Can be Dangerous for your Dog and Cat

2012-02-03T15:25:00.000-05:00

Beware — your Super Sunday party may be dangerous for your pet.

In the article below, Dr. Ernie Ward, founder and President of the Association for Pet Obesity Prevention (APOP), offers tips to pet owners to make sure their Super Sunday party activities don’t end up with an overtime veterinary emergency.

This is a must read for owners of dogs and cats planning to have a party while watching the football game this Sunday.

Links:

Animal Endocrine Clinic March 18 Seminar

2012-01-27T12:58:00.001-05:00

We are pleased to announce our second Animal Endocrine Clinic Seminar:

Animal Endocrinology:

Perils and Pitfalls in the Diagnosis and Treatment of Endocrine Diseases

This seminar will take place on March 18th in Westchester County, NY. AEC Seminars are free and are open to veterinarians and veterinary technicians. The March 18th seminar will deal with several of the most common canine and feline endocrine disorders.

Dr. Mark E. Peterson and Dr. Rhett Nichols will discuss Hyper- and Hypothyroidism, Thyroid and Parathyroid Tumors, Primary Hyperparathyroidism, Idiopathic Hypercalcemia, Diabetes mellitus, Cushing's syndrome, and Hyperaldosteronism.

Further details and the registration form are available here.

Paper Review: Dietary Sugars Intake and Cardiovascular Health. A Scientific Statement From the American Heart Association

2012-01-20T21:06:00.000-05:00

High intakes of dietary sugars in conjunction with the current worldwide pandemic of obesity, hypertension, and cardiovascular disease have heightened concerns about the adverse effects of excessive consumption of sugars in humans. Between 1970 and 2005, average annual availability of sugars consumed increased by 19%, which added 76 calories to Americans’ average daily energy intake.

Excessive consumption of sugars has been linked with several metabolic abnormalities and adverse health conditions, including obesity, hypertension, and cardiovascular disease. In addition, high sugar intake may lead to the development of the metabolic syndrome, as well as increased inflammation and oxidative stress. Finally, excessive sugar intake is associated with shortfalls of essential nutrients.

In view of these considerations, the American Heart Association has published a position paper on this subject, which recommends reductions in the intake of added sugars. Click the following link to read the full paper, which was published in the journal Circulation and is available online (1). It brings home and important message that we need all heed.

My Comments:

Important Role of the Glucose-Insulin Response to Chronic Disease
Many factors influence the body’s glucose response to foods, including the composition of the food (fat, protein, sugar, starch, and fiber content), the method of food processing and preparation, the combination of foods eaten, and physiological factors including age and body composition.

Glucose control is the net effect of metabolic processes that remove glucose from the blood for either glycogen synthesis or energy production and of gluconeogenesis and glycogenolysis, which return glucose to the blood.

The rise in blood glucose after consumption of a carbohydrate triggers the release of insulin and at the same time reduces the secretion of glucagon. Hyperinsulinemia, in turn, appears to be primarily responsible for many of the untoward metabolic problems associated with our present society, including obesity, hypertension, and cardiovascular disease.

Gary Taubes: Good Calories, Bad Calories
The recommendations found in this paper (1) are in agreement with the recommendations Gary Taubes, makes in his book, entitled Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease (2) As I discussed in a recent blog post, in that book Taubes convincingly states that the problem with our diets lies in refined carbohydrates and sugars. He states that refined carbohydrates do harm via their dramatic and long-term effects on insulin, the hormone that regulates fat accumulation, and that the key to good health is the kind of calories we take in, not the number. There are good calories, and bad ones.

Robert Lustig: Sugar: The Bitter Truth
The recommendations found in this position paper are also in agreement with the recommendations Dr. Lustig makes in this lecture called “Sugar: The Bitter Truth” which was posted on YouTube. Dr. Lustig has also published extensively on this topic (3), and is also an co-author on the ADA's position paper on dietary sugar intake (1). Lustig calls fructose a "poison" and compares its metabolic effects with those of ethanol. See my previous blog post for more information.

How Do Dietary Sugars Relate to Animal Nutrition?

Over the past five years, sugar has increasingly been added to some popular brands of dog and cat treats to make them more palatable and profitable (4-7). Like people, these sugars are not needed for our dogs or cats and are best avoided (7). These sugary treats are likely contributing to the rapid rise in obesity in our pets.Sugar is also added to pet foods and treats for a variety of reasons, other than those related to palatability. For example, corn syrup is used as a thickener and to suspend the dough for proper mixing of ingredients, and dextrose is used to evenly distribute moisture throughout a food (4). Sugar has a role in the physical and taste characteristics of many products, helping to mask bitter flavors imparted by acidifying agents, or changing the texture of specific treat types.

Why is the sugar content so high in many pet food treats and some pet foods? Remember that dogs, like humans, are omnivores and both species handle carbohydrates in much the same manner. Cats, on the other hand, are obligate carnivores, so sugars should never be included in their diet, if possible (8).

References:

Johnson RK, Appel LJ, Brands M, Howard BV, Lefevre M, Lustig RH, Sacks F, Steffen LM, Wylie-Rosett J; American Heart Association Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism and the Council on Epidemiology and Prevention. Dietary sugars intake and cardiovascular health: a scientific statement from the American Heart Association. Circulation. 2009;120:1011-1020. http://circ.ahajournals.org/content/120/11/1011.full.pdf+html
Taubes G: Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease 2007.
Lustig RH. The fructose epidemic. The Bariatrician 2009:10-18.
Healthday, December 08, 2010. Those Sweet Pet Treats May Foster Fatness: Sugar appearing more now in dog and cat treats as veterinarian warns of the consequences.
Press Release, Association for Pet Obesity Prevention (www.petobesityprevention.com), June 17, 2010. Kibble Crack – Vet Exposes Sugary Secret of Pet Treats. Sugar is being added to many pet treats contributing to the growing pet obesity epidemic.
Petfood Industry (www.petfoodindusty.com), December 13, 2010. Sugar in pet treats may be contributing to obesity.
The Dog Food Project (www.dogfoodproject.com). Ingredients to avoid.
Kienzle, E. Blood sugar levels and renal sugar excretion after the intake of high carbohydrate diets in cats. The Journal of Nutrition 1994; 124:2563S-2567S.

Why y/d? Reasons Not to Feed Your Hyperthyroid Cat the Hill's y/d Thyroid Diet

2012-01-15T11:55:00.002-05:00

Comprehensive Section on the Pros and Cons of Feeding the Hill's y/d Feline Thyroid Health Diet Added to the Animal Endocrine Website

Dr. Peterson has updated the Animal Endocrine website to provide more in-depth analysis of Hill's new y/d diet being heavily marketed as a treatment for cats with hyperthyroidism. The updated page includes information about the causes of hyperthyroidism, the role of iodine deficiency in this process, and the possible effects of and fallout from feeding y/d diet to a hyperthyroid cat.

There's a difference between using a iodine-deficient diet to address a cat's thyroid hormone levels, and actually treating and abating the disease. Dr. Peterson also has concerns about the nutritional composition of the y/d diet, which, in his opinion, is severely lacking.

Click through to our website to read the full article.

Related links: discussion of the y/d diet on Dr. Peterson's Insights into Veterinary Endocrinology blog.

Dr. Mark Peterson Interviewed for "My Pet World" News Column About Treating a Geriatric Hyperthyroid Cat

2012-01-10T22:33:00.001-05:00

My Pet World: Advice from the Experts

By Steve Dale, Tribune Media Services

Q: The thyroid levels for my 21-year-old cat have recently become elevated, indicative of early stage hyperthyroidism. My veterinarian recommended a prescription diet (Hill's prescription y/d) or transdermally administered methimazole. The veterinarian believes radioactive iodine is a bit too extreme for a cat of this age.

I'm worried that my cat will find the (prescription) diet unpalatable. I'm also concerned about the side effects of transdermal medication. I'm impressed with what I've read about a natural solution called Resthyro. What's your opinion on what I should do for my feisty geriatric cat? -- N.M., Highland Park, IL

A: Dr. Mark Peterson, of New York City, is one of the world's experts on this topic, and helped create the radioactive iodine treatment you mention. Hyperthyroid disease nearly always occurs in older cats. Still, he says he understands that with a cat as elderly as yours, there are limits on what it makes sense to do. "I can see where this cat is no candidate for radioactive iodine," he commented. This treatment typically cures hyperthyroid disease, and is benign, but takes the cat away from home for several days of treatment.

Peterson says he doesn't personally see many adverse reactions to transdermal medication (delivering medication through the skin rather than a pill) for hyperthyroid disease. And he's seen a lot of hyperthyroid cats. Of course, side effects can potentially occur with any medication, but sometimes what you read on the Internet may be skewed. The pill version of methimazole may be just fine, but you no doubt know it can be hard to convince a cat to take a pill, which is why the transdermal might be preferred.

As for the new Hills prescription y/d diet, Peterson says, "It's long-term safety for real use is unknown." He's not absolutely convinced about the diet, and agrees that any diet change might be a problem for a 21-year-old pet. Still, that's a possible option.

As for Resthyro, Peterson said he has not seen much success. If you want to try the "natural" route, ask your veterinarian about L-Carnitine (a nutritional supplement available wherever vitamins and supplements for people are sold). Peterson says if your cat is mildly hyperthyroid, this may the best option worth trying, but do monitor your cat's thyroid levels. Open the capsule and sprinkle the contents on your cat's food or a special treat, such as a bit of chicken. Try starting at 250 mg daily.

Links to Original Article:

Chicago Tribune, http://www.chicagotribune.com/entertainment/books/sns-201112131830--tms--petwrldctnya-a20111215dec15,0,405315.story
Hartford Courant, http://www.gadzoo.com/hartfordcourant/Article.aspx?id=107094

About Steve Dale:
Steve Dale, pet expert and certified dog and cat behavior consultant, reaches more pet owners than any other pet journalist in America. Steve writes a twice-weekly syndicated newspaper column (Tribune Media Services).

He is also a contributing editor at USA Weekend and is the host of two nationally syndicated radio shows, Steve Dale’s Pet World and The Pet Minute, and Steve Dale’s Pet World at WLS Radio, Chicago. To learn more, visit Steve's website at www.stevedalepetworld.com.

What We Can All Learn From Older Animals

2012-01-08T01:32:00.001-05:00

It is not hard to argue that we live in a youth-centric culture, one in which young age and beauty are almost synonymous. And that obsession does not end with humans. Puppies and kittens melt hearts; images and videos of baby animals flood the Internet. But rarely does an image of an animal in old age ignite the same interest and adoration.

Mortality and aging are never an easy subject to discuss. Finding meaning and joy in life in the face of physical limitations and challenges.

In an unusual project, Isa Leshko, a fine-art photographer who lives in Philadelphia, set out to capture glimpses of animals at a time when they rarely attract much admiration or media attention — in their twilight years (1). The photographs, part of a series called “Elderly Animals”, are intimate and at times gripping (2).

You can view more photos at Ms. Leshko’s Web site (3), which also contains a short documentary by Mark and Angela Walley about her “Elderly Animals” project (4).

Links:
1. The New York Times Well Blogs. What we can learn from old animals. December 29, 2011.
2. http://isaleshko.com/elderly-animals/
3. http://www.isaleshko.com/index.cfm
4. http://vimeo.com/29632448

Dr. Mark Peterson Receives Prestigious Award from The New York State Veterinary Medical Society

2012-01-03T09:11:00.000-05:00

On December 8, 2011, Dr. Mark Peterson was awarded the President's Citation from the New York State Veterinary Medical Society.

This award to given to Mark in recognition and appreciation of his outstanding guidance and success in his role in organizing the speakers for the program of the annual New York State Veterinary Conference.

Dr. Mark Peterson awarded the President's Citation by Dr. Eric Bregman (on right),
the current President of the New York Veterinary Medical Society

The New York State Veterinary Conference provides three days of high-quality continuing education to hundreds of veterinarians, veterinary technicians, and students from across New York State and throughout the Northeast. This conference is hosted by the New York State Veterinary Medical Society and Cornell University's College of Veterinary Medicine and is held each fall in Ithaca, New York.

Documentary of Pet Food Industry — "Pet Food: A Dog's Breakfast"

2011-12-29T22:27:00.002-05:00

We love our pets, but do we really know what we're feeding them? My last post discussed how to better interpret a pet food label.

Now, if you haven't already seen it, I've included a link to a must-see documentary about the pet food industry entitled, Pet Food: A Dog’s Breakfast (1). But be warned— this film is almost 45 minutes long.

This film takes the lid off the can to discover what’s going on inside the $16 billion pet food industry. The program, produced by Yap Films, an independently owned production company (2), was originally televised by the Canadian Broadcasting Corporation in August of 2009. Its production was sparked by the 2007 pet food recall in which tens of thousands of cats and dogs in North America became sick after being fed tainted pet food.

The film supposedly had been scheduled to air on CNBC earlier this year but was canceled without explanation. But now, interestingly, it has resurfaced on YouTube for all of us to see.

So when you see the ads about pet food, claiming that the food is "healthy, has all the essential nutrients, and is best for your pet" we should stop and wonder — is all that marketing really true? This documentary uncovers part of the truth. Yet there is much more to learn. After watching this documentary, however, your eyes should open up and you may no longer blindly trust the commercial pet food companies. I no longer do.

But what to feed? Home-made, raw? I certainly don't have all the answers. But if you have the time, home-prepared diets may offer the best means to feed your pet a healthy whole food diet (3-5).

Links:

YouTube. Pet Food: A Dog's Breakfast
Yapfilms.com
Strombeck's Home-Prepared Diets For Dogs and Cats: Safe, Balanced, Complete & Up-to-Date. Dogcathomeprepareddeit.com.
Schenck P. Home-Prepared Dog & Cat Diets. Second Edition, Wiley-Blackwell, 2010.
Pierson LA. Making cat food. Catinfo.org.

Pet Food Labels: Reading Between the Lines

2011-12-23T12:18:00.000-05:00

We can all be passionate about choosing the best food for our dogs and cats. With thousands of pet food products on the market, how do pet owners make the best choice?

Pet food labels are a good place to start. Understanding the label information can help us make informed decisions about the food we feed their pets. Knowing how to read a pet food label can help answer questions, serve our pets’ nutritional needs, and optimize patient care.

This easy-to-read article addresses how to read a pet food label discusses the key issues you should be familiar with before you go shopping for pet food in order to make the best choice for your dog or cat.

Smith JM. Pet food labels: Reading between the lines. Veterinary Technician. September 2011; Volume 32, Number 9. Online at www.vetlearn.com.

Dangers of Holiday Snacking for Your Diabetic Dog or Cat

2011-12-22T15:02:00.000-05:00

During the winter holidays, the average person tends to gain weight because of the parties and snacking on delicious goodies. Our dogs and cats are keen to these festivities.

Our pets are often times smarter than we are when it comes to food. An otherwise normal pet can become a diabolical genius when it comes to begging. Your pet's pleading warm brown eyes that seem to penetrate to your very soul and can sway your better judgment. But what may be a "little bite" to someone the size of a human is much more to a small dog or cat!

More of a Problem in the Diabetic Dog or Cat

Although feeding treats to a normal dogs and cats may just lead to weight gain, the issue is much more important in your diabetic pet.

Dogs tend to be keenly aware of all the goodies and baked goods in the home. All the sugar in such special holiday treats can be very detrimental to diabetic control. Giving a high-sugar treat in the middle of the day is likely to cause an unwelcome glucose spike. If you must give in, choose a low-sugar healthy snack and give it with the dog's meal. Of course, if you are giving a treat, cut back appropriately on the amount of regular food given to your dog.

These snacking situations may be less troubling for cats. Cats are usually drawn toward the ham and turkey and not so much the sweets and breads we find in our homes over the holidays. Cats do not have the taste receptors for sweetness so they cannot taste sugars (1-3). Cats tend to be nibblers and are often placed on a longer-acting insulin. A nibble here and there of a low carb bit of meat isn't typically a problem for your diabetic cat.

Chocolate is Toxic

Make sure that any holiday chocolates are kept away from where your pet can reach them. Even small amounts of chocolate can result in poisoning in dogs and cats (4).

The toxic principles in chocolate are the methylxanthines, theobromine and caffeine. Although the concentration of theobromine in chocolate is 3-10 times that of caffeine, both constituents contribute to the clinical syndrome seen in chocolate toxicosis. Listed below are the total methylxanthine concentration of commonly used chocolate products:

Dry cocoa powder ~800 mg/oz (28.5 mg/g)
Unsweetened (baker’s) chocolate ~450 mg/oz (16 mg/g)
Semisweet chocolate and sweet dark chocolate ~150-160 mg/oz (5.4-5.7 mg/g),
Milk chocolate ~64 mg/oz (2.3 mg/g)
White chocolate is an insignificant source of methylxanthines.

Serious poisoning happens frequently in dogs and cats who metabolize methylxanthines much more slowly than human; they can easily consume enough chocolate to be poisoned (5-6).

The most common victims of theobromine poisoning are dogs, for which it can be fatal. The toxic dose for cats is even lower than for dogs. Again, cats are less prone to eating chocolate since they are unable to taste sweetness (1-3).

In general, mild signs (vomiting, diarrhea, polydipsia) may be seen in dogs ingesting 10-20 mg/kg, cardiotoxic effects may be seen at 40-50 mg/kg, and seizures may occur at doses ≥60 mg/kg (5-7). One ounce of milk chocolate per pound of body weight is a potentially lethal dose in dogs.

A typical 20 kg (44 lb) dog will normally experience intestinal distress after eating less than 240 g (8.5 oz) of dark chocolate, but won't necessarily experience toxicity unless it eats at least half a kilogram (1.1 lb) of milk chocolate. Large breeds (>100 lbs) can safely consume chocolate in limited quantities, but care must still be taken, as they can safely eat only about a quarter the amount a human can, and should not be intentionally fed it.

References:

Li X, Wang H, Cao J, et al. Pseudogenization of a sweet-receptor gene accounts for cats' indifference toward sugar. PLoS Genetics 2005;1:27-35.
Li X, Li W, Wang H, et al. Cats lack a sweet taste receptor. Journal of Nutition 2006;136:1932S-1934S.
Biello, David (August 16, 2007). Strange but True: Cats Cannot Taste Sweets. Scientific American. Retrieved July 28, 2009.
Smit HJ. Theobromine and the pharmacology of cocoa. Handbook of experimental pharmacology 2011;(200):201-234.
Stidworthy MF, Bleakley JS, Cheeseman MT, et al. Chocolate poisoning in dogs. Veterinary Record 1997;141:28. http://www.ncbi.nlm.nih.gov/pubmed/9248028
Gunning ME, den Hertog E, van Velsen NF, et al. Chocolate intoxication in dogs. Tijdschrift voor Diergeneeskunde. 2010;135:896-899.
The Merck Veterinary Manual (online edition). Chocolate. www.merckvetmanual.com

Sugar: The Bitter Truth by Robert Lustig, MD

2011-12-17T00:45:00.000-05:00

In my last post, I reviewed the book by Gary Taubes, entitled Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease. In that book, Taubes convincingly states that the problem with our diets lies in refined carbohydrates and sugars. He states that refined carbohydrates do harm via their dramatic and long-term effects on insulin, the hormone that regulates fat accumulation, and that the key to good health is the kind of calories we take in, not the number. There are good calories, and bad ones.

Robert H. Lustig, MD, Professor of Pediatrics in the Division of Endocrinology, University of California San Francisco has published and lectured extensively on the damage caused by sugary foods, especially the sugar fructose (1-8). He argues that ingestion of too much fructose and and not enough fiber appear to be cornerstones of the obesity epidemic in the US and result of the developed countries through their effects on insulin.

On May 26, 2009, Dr. Lustig gave a lecture called “Sugar: The Bitter Truth” which was posted on YouTube the following July and has become a viral success with over a million viewings. In this lecture, Lustig notably calls fructose a "poison" and compares its metabolic effects with those of ethanol. This lecture is a excellent and through review of this topic, but be warned — this is an hour and a half lecture! But I think that you will find it worth your time— it may change the way you life and eat.

Sugar: The Bitter Truth

In the second lecture presentation, Dr. Lustig, discusses "sugar addition" and explores methods we can all use to reduce sugar consumption.

Interventions to Reduce Sugar Consumption

References:
Dr. Lustig's Papers on Fructose

Lustig RH, Sen S, Soberman JE, et al. Obesity, leptin resistance, and the effects of insulin reduction. International Journal of Obesity and Related Metabolic Disorders 2004;28:1344-1348.
Lustig RH. Which comes first? The obesity or the insulin? The behavior or the biochemistry? The Journal of Pediatrics 2008;152:601-602.
Johnson RK, Appel LJ, Brands M, et al. Dietary sugars intake and cardiovascular health: a scientific statement from the American Heart Association. Circulation 2009;120:1011-1020.
Lustig RH. The fructose epidemic. The Bariatrician 2009:10-18.
Lim JS, Mietus-Snyder M, Valente A, et al. The role of fructose in the pathogenesis of NAFLD and the metabolic syndrome. Nature Reviews Gastroenterology & Hepatology 2010;7:251-264.
Lustig RH. Fructose: metabolic, hedonic, and societal parallels with ethanol. Journal of the American Dietetic Association 2010;110:1307-1321.
Nguyen S, Lustig RH. Just a spoonful of sugar helps the blood pressure go up. Expert Review of Cardiovascular Therapy 2010;8:1497-1499.
Garber AK, Lustig RH. Is fast food addictive? Current Drug Abuse Reviews 2011;4:146-162.

How Do Dietary Sugars Relate to Animal Nutrition?
Over the past five years, sugar has increasingly been added to some popular brands of dog and cat treats to make them more palatable and profitable (1-3). Like people, these sugars are not needed for our dogs or cats and are best avoided (4). These sugary treats are likely contributing to the rapid rise in obesity in our pets.

Sugar is also added to pet foods and treats for a variety of reasons, other than those are related to palatability. For example, corn syrup is used as a thickener and to suspend the dough for proper mixing of ingredients, and dextrose is used to evenly distribute moisture throughout a food (1). Sugar has a role in the physical and taste characteristics of many products, helping to mask bitter flavors imparted by acidifying agents, or changing the texture of specific treat types.

Dr. Lustig's work also emphasizes the importance of nutrition as part of treatment of all medical disease (see above). His work in this field has been both compelling and eye-opening, and I am now thinking more critically about the unnecessarily high sugar content in pet food treats and some pet foods. Remember that dogs, like humans, are omnivores and both species handle carbohydrates in much the same manner. Cats, on the other hand, are obligate carnivores, so sugars should never be included in their diet, if possible (5).

Animal References:

Healthday, December 08, 2010. Those Sweet Pet Treats May Foster Fatness: Sugar appearing more now in dog and cat treats as veterinarian warns of the consequences.
Press Release, Association for Pet Obesity Prevention (www.petobesityprevention.com), June 17, 2010. Kibble Crack – Vet Exposes Sugary Secret of Pet Treats. Sugar is being added to many pet treats contributing to the growing pet obesity epidemic.
Petfood Industry (www.petfoodindusty.com), December 13, 2010. Sugar in pet treats may be contributing to obesity.
The Dog Food Project (www.dogfoodproject.com). Ingredients to avoid.
Kienzle, E. Blood sugar levels and renal sugar excretion after the intake of high carbohydrate diets in cats. The Journal of Nutrition 1994; 124:2563S-2567S.

Book Review: Good Calories, Bad Calories

2011-12-12T12:48:00.001-05:00

For decades we have been taught that fat is bad for us, carbohydrates better, and that the key to a healthy weight is eating less and exercising more. Yet with more and more people acting on this advice, we have seen unprecedented epidemics of obesity and diabetes.

Gary Taubes, a correspondent for Science magazine, contributed to the Atkins Diet craze with his New York Times article several years ago, "What If It’s All Been a Big Fat Lie?." He then spent the past several years expanding on that article, and the result is Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease, a book of some 600 pages (nearly 70 of which make an extensive list of references).

In this book, Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease, Taubes argues persuasively that the problem lies in refined carbohydrates (e.g., white flour, sugar, easily digested starches) and sugars. He states that refined carbohydrates do harm via their dramatic and long-term effects on insulin, the hormone that regulates fat accumulation, and that the key to good health is the kind of calories we take in, not the number. There are good calories, and bad ones.

Good Calories
These are from foods without easily digestible carbohydrates and sugars. Examples in include meat, fish, fowl, cheese, eggs, butter, and non-starchy vegetables. Taubes claims that these foods can be eaten without restraint.

Bad Calories
These are from foods that stimulate excessive insulin secretion and so make us fat and increase our risk of chronic disease—all refined and easily digestible carbohydrates and sugars. The key is not how much vitamins and minerals they contain, but how quickly they are digested. Therefore, apple juice or even green vegetable juices are not necessarily any healthier than soda.

Examples of "bad calories" according to Taub include bread and other baked goods, potatoes, yams, rice, pasta, cereal grains, corn, sugar (sucrose and high fructose corn syrup), ice cream, candy, soft drinks, fruit juices, bananas and other tropical fruits, and beer.

Major Critical Points of Good Calories, Bad Calories
Taubes has several overarching themes. He contends, for example, that eating refined carbohydrates is what makes you obese, and that refined carbohydrates contribute to many of what used to be called "diseases of civilization" such as heart disease, which seems to have been less common in traditional cultures that ate less processed food, including Northern cultures that ate almost exclusively meat. (These arguments are still controversial, although new evidence continues to support them.)

With precise references to the most significant existing clinical studies, he convinces us that there is no compelling scientific evidence demonstrating that saturated fat and cholesterol cause heart disease, that salt causes high blood pressure, and that fiber is a necessary part of a healthy diet. Based on the evidence that does exist, he leads us to conclude that the only healthy way to lose weight and remain lean is to eat fewer carbohydrates or to change the type of the carbohydrates we do eat, and, for some of us, perhaps to eat virtually none at all.

Overall, this groundbreaking book, the result of seven years of research in every science connected with the impact of nutrition on health, award-winning science writer Gary Taubes shows us that almost everything we believe about the nature of a healthy diet may be wrong.

How Does This Book Relate to Animal Endocrinology?
It is obvious that many aspects of nutritional biochemistry and metabolism involve endocrinology and metabolism (regulation of insulin secretion and fat production). Remember that we as humans are also animals, and as omnivores, our nutritional needs are fairly close to dogs and other omnivores. Cats, on the other hand, are obligate carnivores, so their nutritional needs are quite different (i.e., high protein requirements but no need for carbohydrates at all).

This book also emphasizes the importance of nutrition as part of treatment of all endocrine disease, as well as all medical disease in general. The nutritional content of food has been something I have been focusing on recently, both for myself personally and also as a veterinary endocrinologist interested in furthering my understanding of animal health and medicine. Taubes' work in this field has been both compelling and eye-opening, and I am now thinking more critically about the unnecessarily high carbohydrate content of pet food (especially for the carnivorous cat).

Overheard on the Web: Hypurrcat Recommended on the Feline Hyper-T Yahoo Group

2011-12-07T10:34:00.000-05:00

Below is a post we found on the Hyperthyroid Cat Yahoo Group, which is the original support list for people whose cats have been diagnosed as hyperthyroid or hypothyroid. Founded in 1999, the group has over 2400 members.

This is a great resource for anyone whose cat has been diagnosed with hyperthyroidism or hypothyroidism. The group is full of caring and supportive people who are more than willing to share their experience.

I-131 In NYC?

I joined the group about two weeks ago after we got a hyperthyroid diagnosis for my 14-year old kitty. She's started the methimazole, which seems to be going well. She's hard to give pills to, and HATES having her ears messed with, so we opted to try the chicken flavor "chewies" — which she scarfs right down. What a relief!

Now we're starting to think about the I-131 procedure. Our vet said they have a few places that they can recommend, and we'll get more details from them when we go back for the first checkup in a week or two, but thought we might start doing research now. We live in NYC, so that would be best for us.

I know Dr. Peterson has a clinic in Manhattan and is highly thought of on this group, and we'll definitely be looking into that option.But I was hoping there are others on the list who might offer their experiences with any places in NYC.

Sampson Thunderpaws, a 12-year, male Maine Coon cat treated at Hypurrcat

Response: I-131 In NYC?

Sampson Thunderpaws went to Dr. Peterson's clinic in Bedford Hills, NY about 3 weeks ago. If the clinic in Manhattan is anything like the one in Bedford Hills, Suki will be thoroughly spoiled by the time she gets home.

Dr. Peterson tailors each I-131 dosage to the cat and his/her needs rather than give a set dosage per cat as some of the clinics do.
There is a vet tech in with the cats all day, and I caught her feeding Sampson (there are webcams in every "condo") after 8 pm one night.
They answered all my emails promptly — and I sent a "few". :) Sampson was hand-fed when he didn't want to eat and they gave him additional cooked pieces of chicken to tempt him.
They have lots of things for the cats to watch — cat TV and real, live Gerbils in cages just outside their "condos".
Oh — and they brought him a gerbil in a ball to keep his hunting skills sharp. LOL! (A vet tech was there every minute so no gerbils were harmed in the process.)

Dr. Peterson is the man for hyperthyroidism with 30-plus years experience, so that is the real reason we went there—but I couldn't have asked for a better experience for Sampson. Which made it easier for me to leave him there.

Peace, Plenty and Purrs,
Memory and Sampson Thunderpaws

Sampson Thunderpaws, a hyperthyroid cat treated at Hypurrcat

Rosie, the Bodega Cat: Epilogue #2

2011-11-28T14:00:00.000-05:00

Hi there,

"Rosie" here again... it's been awhile since I last wrote to you... see my previous posts here to get my background.

November 2, 2011: The Story of Rosie, the Hyperthyroid Bodega Cat
November 7, 2011: Rosie, the Hyperthyroid Bodega Cat: an Epilogue

This must be heaven — Cat Heaven, that is!

My new home is beautiful and comfortable beyond my wildest dreams. Yes, I often dreamed of going back to a beautiful apartment again, but this place is absolutely divine, or, as they say: it's the cat's meow! Unbelievable!

Now I have two beds. One is in the living room den area, and one in Gladys' bedroom. I have breathtaking views of the city from a very high, unobstructed viewpoint. I also have lots of toys, and a shiny new feeder and water bowl. There's lots of windows, and a balcony with a bird feeder. So I'm having fun watching the birds again, just like I did during my stay at the Hypurrcat treatment facility at at the Animal Endocrine Clinic to get my hyperthyroidism cured!

This is all very hard to believe —maybe I have died and gone to Cat Heaven! Pinch me, please! But I'm here, and I love it. And Gladys is very happy to have me here, and in her life.

Sometimes I miss my old life at the bodega, but that feeling doesn't last long! Why should it? I'm now "Rosie, the Celebrity Penthouse Queen Cat"!

That's a long title for a little 9-pound "Rosie," but it's true, and I love it! Gladys and I are going to be so happy together. It's like a fairytale dream come true, and little "Rosie" is the main character in this dream life I'm leading now. I hope you enjoyed hearing about this new and exciting change for "Rosie" and Gladys!

That ends my "Rosie" story for now, because I'm going to be really happy and content in my new home for the rest of my life. Wouldn't you be?

Life is good... And I feel much better now that my hyperthyroidism has been cured!

XOXO
Rosie

Dr. Peterson Interviewed about Hyperthyroidism on PetLifeRadio

2011-11-14T19:35:00.001-05:00

PetLifeRadio.com is the #1 pet podcast radio network, featuring weekly 30-minute talk shows hosted by the most well-known pet experts.

On Wednesday, November 10, 2011, Dr. Mark Peterson was interviewed by the Pet Doctor (Diane Levitan, DVM) concerning the latest information about hyperthyroidism in cats. To listen to this interview, click on this link to go to "The Pet Doctor" page on PetLifeRadio.com.

Episode 129:

Hyperthyroidism In Cats—A Very Common Problem with Many Options—A Must Listen Show for Anyone with Cats Over 9 Years of Age

Excerpt:
In this episode, we discuss a very common problem that happens in older cats — it’s a disease that has become much more common since the 1980s and is something that has been a difficult one to figure out. There turns out to be a good reason why older cats eat like crazy but still become very thin, they drink tons of water all the time, and their hyperactive behavior keeps their owners up at night!

It is very likely that the symptoms are due to an over-active thyroid gland, also called hyperthyroidism. There is a great deal of information on feline hyperthyroidism that has been learned in the last 20 years—what is it from? Why is it so common? What are the long-term effects on our cats and how can we prevent it?

Today we are extremely privileged to have Dr. Mark Peterson, a veterinary endocrinologist and the first in the world to publish information on this ever so common illness in cats, hyperthyroidism. He is known worldwide for his research and contributions to veterinary medicine and veterinary endocrinology and is the go to person in the world when it comes to feline hyperthyroidism and many other veterinary endocrinology topics.

Learn more about Dr. Peterson by listening and also check out his web site: www.animalendocrine.com.

Click here to listen to the interview or download it to your computer. This is a 30-minute interview. Information on nutritional management using Hill's y/d diet is discussed in the last 5 minutes.

World Diabetes Day

2011-11-14T08:47:00.001-05:00

Today, November 14th, is World Diabetes Day

World Diabetes Day is the primary global awareness campaign of the diabetes mellitus world and is held on November 14 of each year (1). It was introduced in 1991 by the International Diabetes Federation and the World Health Organization in response to the alarming rise of human diabetes around the world.

World Diabetes Day is a campaign that features a new theme chosen by the International Diabetes Federation each year to address issues facing the global diabetes community. While the campaigns last the whole year, the day itself marks the birthday of Frederick Banting who, along with Charles Best, first conceived the idea which led to the discovery of insulin in 1922 (2).

Charles H. Best and Frederick G. Banting (on right) with one of the diabetic dogs used in their studies of insulin

This year's campaign theme is diabetes education and prevention. In the past four years alone, diabetes rates among dogs in the U.S. have increased roughly 33% among dogs and 16% among the nation's cat population, per a national analysis of pet health (3). This emphasizes the importance of this disorder in our cats and dogs as well as human patients with diabetes.

To successfully manage diabetes in animals, one must understand the disease and monitor and provide daily treatments to the cat or dog with diabetes. Treatment involves a combination of weight loss (if obese), diet, and insulin injections generally twice daily.

References

World Diabetes Day – Official website.
Frederick Grant Banting (1891-1941), codiscoverer of insulin. Journal of the American Medical Association 1966;198:660-661.
Washburn L. Human health risks on the rise in animals. The Record. April 20, 2011.

Update on the Perilous Situation of Dogs & Cat Exposed to Radiation Fallout from the Fukushima Nuclear Disaster

2011-11-11T13:28:00.001-05:00

Thousands of dogs and cats were abandoned after an earthquake damaged Japan's Fukushima nuclear plant, and continued radiation concerns have stymied recovery efforts.

Although the radioactive material the dogs and cats were exposed to during the disaster should be cleared by the their bodies by now, the animals may have incurred permanent damage according to Timothy Mousseau, a professor of biological sciences at the University of South Carolina, who has extensively studied animals exposed to radiation.

Mousseau's research has found significant genetic damage and breakages in chromosomes among animals exposed to radiation in and around Chernobyl, a situation similar to that of Fukushima. Developmental abnormalities, tumors, and species decline and extinction have also been attributed to radiation exposure in that area, and are likely to also occur in Japan.

To read the story and see photos of dogs and cats living near the Fukushima nuclear plant, which was published in PBS Newshour, the Rundown Blog, click this link.

For an overall update on the situation in Japan, click this link to view a podcast more about the persistent radiation readings around the Fukushima area on the PBS Newshour website.

How Dogs Evolved from Wolves to Become Domesticated

2011-11-09T13:58:00.001-05:00

Dogs have aided humans for thousands of years. Man's best friend has provided protection, companionship and hunting assistance since the days of the earliest human settlements. But how and when dogs evolved from wolves is a matter of debate.

In his book "How the Dog Became the Dog — From Wolves to Our Best Friends," naturalist Mark Derr writes that wolves who attached themselves to human hunting parties for survival ultimately evolved into today's dog species.

Groups of dogs that socialized well with people split off from the wilder dogs, mated and led to the domesticated species we have today.

If you would like to read or listen to an interview with Mark Derr on National Public Radio about his book, click on this link to go to the NPR website.

Rosie, the Hyperthyroid Bodega Cat: an Epilogue

2011-11-07T17:03:00.000-05:00

Hi Everyone,

This is "Rosie" again. You won't believe this next chapter in my life!

You remember that beautiful lady Gladys who I'm crazy about? Of course, you already know she's crazy about me, too. We've bonded — it's mutual love!

Well, after my delightful 5-day stay at Dr. Peterson's Hypurrcat facility at the Animal Endocrine Clinic where I was treated for my hyperthyroid condition, it was time to go home.

I had a ball at Dr. Peterson's, living a life of luxury. But now, instead of going back to the bodega, they decided it was best for me to go home with Gladys!

And the amazing thing is that Gladys lives in a beautiful penthouse apartment in a very posh section of NYC. I'm very very excited and happy about this turn of events, because after all the heartbreak of losing my first mistress, now I'm going home to a gorgeous penthouse apartment, living in total luxury.

Although this does seem a fitting domicile for a cat as charming as myself — I still can't believe how lucky I am! I'm going home to my new life tomorrow, and I can hardly wait!

I promise to keep you posted and fill you all in on how everything turns out.

PS: Oh, another thing. Dr. Peterson just called Gladys and told her that the radioiodine treatment worked. My hyperthyroid condition is completely cured!

The Story of Rosie, a Hyperthyroid Bodega Cat

2011-11-02T14:58:00.000-04:00

Hello my name is "Rosie."

I'm a cat—but not your everyday, ordinary, run-of-the-mill cat, mind you. I am, I must say, a beautiful, all-white, Turkish/Persian long-hair with stunning yellow to green eyes. People are easily attracted to me —not just for my beauty, but also for my personality. I am very special in everyone's eyes!

Let me tell you about my past:

As a kitten, I was cared for by a very lovely lady who lived in a large, lovely apartment in New York City. I was very happy with her for many years, but then, a tragic turn of events made me an orphan. My mistress suddenly passed away, and I was all alone in the world! But a kindly doorman in my building knew me well, and he decided to take me into his home. He has a lovely wife and two children, so I thought: "This is going to be a great home for me." But it was not to be— his children were very allergic to cats, and even though they loved me a lot, I could not continue to stay there.

So the kindly doorman started his search for a new home for me.

There is a small bodega in the area run by a nice man and his two sons (for those of you who live outside of the City, a "bodega" is a small deli and convenience store; there's one on every block in New York City). The present owner has run this bodega for over 30 years. They all work long shifts, 7 days a week. He has other family overseas, so he and his sons rotate, one of them going back to visit the family about every 6 months.

Anyway, I was introduced to the owner and two sons, and they immediately loved me and adopted me as their own. So for the last 3 years, I have been "Rosie the Bodega Cat." I like it there. I see and meet many people, all day long, every day. They treat me very well, feed me, and look after me in all ways, as best as they know how. At night I guard the store, and I'm a good security guard!

Then, in the June of 2011, a wonderful thing happened to me, and my life changed once again.

A very beautiful blonde lady named Gladys, who came to the bodega occasionally, noticed me and took a shine to me. Before long, she fell in love with me and I love her very much. Gladys has been looking out for me in all respects. She comes to see me 2 or 3 times a day. As soon as I hear her voice, I come right out to see her, and she plays with me and pets me. Sometimes she'll give me a treat —I like that!

One day Gladys decided that I needed some grooming. So with the blessing of the owners of the bodega, I was to go to a big animal hospital in the neighborhood to get a bath and grooming. A luxury day at the cat spa! So one morning, I went with Gladys a few blocks to the animal hospital.

It all sounded simple enough, but alas—complications developed. It seems as though I needed a slight sedative so my groomer would have no behavioral problem while grooming me. Unfortunately, during my grooming, another big problem was found. I had developed a rather large, ugly black tumor on my stomach, near my mammary glands.

So now I needed lots of professional veterinary care. A simple grooming turned into a major medical problem for me. Simply put, I would have to be sedated and put under anesthesia, so that they could remove that ugly tumor and have it checked for malignancy—you know: cancer!

Dr. Peterson examining Rosie

So my surgery was done on a Friday. My operation went very well. My doctor was excellent, and she also loved me very much. To tell you the truth, all the hospital employees loved me to death. They came by my kennel area often, and took me out, made a fuss over me, and played with me, and gave treats too. Not bad, luxurious living for a bodega cat!

I stayed at the hospital for about 10 days for care and observation. I also had to wear a silly neck collar, so I could not reach my tummy and cause a healing problem by licking the incision. We cats love to groom ourselves — grooming and sleeping are big parts of our day!

I've been dubbed "Rosie, the Celebrity Cat," not just there at the hospital, but in the whole neighborhood! Look out, Hollywood, here I come! The first animated "Rosie the Cat" movie is just around the corner. By the way, good news! The tumor was benign — you know, no cancer!

Oh, I forgot to mention another complication. The doctor gave me a very comprehensive pre-operative exam. As a result, it was discovered that I had a problem common to many cats: hyperthyroidism. That's a mighty big word for a small, beautiful, white, 9-pound cat, don't you think?

So after another grooming, I went home for about 6 days to complete my recovery. Gladys then took me to a specialty hospital: Dr. Mark E. Peterson's Animal Endocrine Clinic up on West 100th Street. At the AEC, Dr. Peterson has a Hypurrcat treatment facility where he treats cats specifically for hyperthyroidism with radioiodine. How about that name —Hy-purr-cat? By the way, I purr a lot because I'm a very happy, content cat!

So last Sunday, I met Dr. Peterson, who turns out to be the world's expert on my condition, having treated over 10,000 cats over his 30 year career. Now, Rosie, the Celebrity Cat will be number 10,001! He carefully examined me but spent a great deal of time petting me, which I actually enjoyed (he is very gentle!). As Dr. Peterson explained to Gladys, there are no side effects, no pain, just one little injection! And it's a very effective treatment! And he was right: the treatment consisted of a little injection under the skin, and it really didn't hurt a bit.

Rosie in her condo at Hypurrcat

Wow, can you believe all of this? I can't, but guess what? I'm enjoying my stay here! First, my accommodations are really nice. I'm in the Penthouse condo, high above everyone so I have a great view. I have my own large personal condo, of course, with hiding boxes, perches, litter box, alcoves and good ventilation!

Now get this, folks: I also have my own flat-screen TV that plays relaxing nature videos! I also have a window view of the rear courtyard, where I can see squirrels, and birds, eating from feeders! Dr. Peterson and his staff are really nice to me here — I can eat as much as I want and I get lots of attention.

I can't believe all this is happening to "Little Rosie, the Bodega Cat!" What a life! I thought you would like to hear my story. I hope you like it!

But most of all, I thank you very much Gladys for your deep love and care for me! Thankfully you got over your cat allergy just in time to look after me so lovingly. You are the best, the best thing that has ever happened to me! I love you to pieces!

XOXO

Rosie

PS: After I leave Hypurrcat (I don't really want to go — the accommodations here are great and I love watching the birds), I don't know where I'll end up, but I promise to keep you all updated as my saga continues.

Treating Cats with Hyperthyroidism: Antithyroid Drugs

2011-10-23T11:32:00.000-04:00

In cats, hyperthyroidism can be treated in four ways — chronic administration of an antithyroid drug, surgical thyroidectomy, radioactive iodine (131-I), or lifelong feeding of an ultra-low iodine diet.

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated heart or kidney diseases or other major medical problems, availability of a skilled surgeon or radioiodine treatment facility, and owner's preference (1-4).

In this post, I’m going to discuss the use of antithyroid drugs for treating cats with hyperthyroidism. This is the most common means that veterinarians use to treat this common condition, so let’s start by discussing the pros and cons of this form of treatment.

The Advantages and Disadvantages of Antithyroid Drugs

Advantages
Chronic management with antithyroid drugs is a practical treatment option for many cats with hyperthyroidism, and offers many advantages. Medical management requires no special facilities and can be prescribed by all veterinarians (1-4). These drugs cause a rapid fall in serum thyroid hormone levels (i.e., the high serum T4 normalizes within 1-3 weeks), which may be desirable in severely affected hyperthyroid cats (5,6).

Anesthesia is avoided, as are the surgical complications associated with thyroidectomy (I’ll be discussing surgical treatment in my next post). In contrast to surgery or radioiodine treatment, hospitalization is not required with medical treatment. Finally, the initial, upfront costs of antithyroid drugs is much less than with either surgical or radioactive iodine treatment.

Disadvantages
Long-term medial management also has many disadvantages. This form of treatment is not curative, is highly dependent on owner and cat compliance, and requires regular biochemical monitoring to ensure the efficacy of treatment (1-4). Side effects are common, occurring in up to 20% of cats (1-6). Even though the initial cost of medical treatment may be far less initially, the cost of ongoing monitoring over a period of months to years can exceed that of thyroidectomy or radioiodine therapy.

These antithyroid drugs also come with other drawbacks. Since they block thyroid hormone synthesis but do not destroy the cat’s thyroid tumor, these drugs never cure the hyperthyroidism and relapse will always occur if daily medication is discontinued (1-6). Most importantly, the benign thyroid tumor — which is present in all cats with hyperthyroidism (7,8) — continues to grow and, after many months, may transform from adenoma to thyroid carcinoma in some cats (9).

Long-term medical management is best reserved for cats of advanced age or for those with concurrent diseases, and for when owners refuse either surgery or radioactive iodine. In addition to long-term treatment, medical management is also advised prior to surgical thyroidectomy to decrease the metabolic and cardiac complications associated with hyperthyroidism. Short-term medical management is often recommended as trial therapy to determine the effect of restoring euthyroidism on kidney function, especially in cats with suspected chronic kidney disease (1-4,10).

Methimazole and Carbimazole: The 2 Antithyroid Drugs

The two drugs methimazole and carbimazole are commonly recommended for managing cats with hyperthyroidism (1-6). A related drug, propylthiouracil, often used in human medicine, is not recommended for cats because of a high incidence of serious adverse reactions —especially anemia and bleeding problems (11).

Methimazole blocks thyroid hormone synthesis by inhibiting thyroid peroxidase, an enzyme involved in the oxidation of iodide to iodine, incorporation of iodine into thyroglobulin, and coupling of tyrosine residues to form T4 and T3 (12). Methimazole does not block the release of preformed thyroid hormone, so there is a delay of 1 to 3 weeks before serum T4 concentrations return to normal after initializing therapy (5, 6).

Carbimazole is a pro-drug of methimazole. That means that after oral administration of carbimazole, it is almost immediately converted to methimazole by the cat (13). So basically, it turns out that carbimazole and methimazole end up being the same drug (1-4).

Different Drug Formulations of Methimazole & Carbimazole

Methimazole Tablets
Methimazole is specifically licensed for treatment of feline hyperthyroidism both in the USA and Europe as 2.5- and 5-mg tablets (Felimazole, Dechra Veterinary Products). It is also available as a generic and brand name drug for human use (Tapazole). For most hyperthyroid cats, a starting dose of 1.25 mg to 2.5 mg methimazole is administered twice daily is recommended (1-4,14).

Carbimazole Tablets
Carbimazole is available for human use in many European countries (brand name, NeoMercazole), Australia and Japan (2-4). This drug is not available as a licensed drug in the USA, but it is available though compounding pharmacies.

As noted above, carbimazole exerts its antithyroid effect through immediate conversion to methimazole when administered orally (13). Serum concentrations of methimazole achieved after carbimazole administration are less than after a similar weight of methimazole such that a 5-mg dose of carbimazole is approximately equal to 3 mg of methimazole (3,6). Because of that, a starting dose of regular carbimazole of 2.5 mg to 5 mg twice daily is commonly recommended for restoring euthyroidism (2-4,6).

Carbimazole is often touted as having a lower incidence of adverse reactions such as vomiting and anorexia (6,15). This may be because it is tasteless whereas methimazole has a bitter taste (3,6). However, Felimazole, as licensed for veterinary use, is sugar-coated; provided the tablet is not crushed, the bitter taste is presumably avoided.

Carbimazole Tablets (Controlled-Release)
A controlled-release formulation of carbimazole (Vidalta, Intervet Schering Plough) is licensed for cats in Europe for once daily administration (16,17). This formulation is not available in the USA.

Administration of this drug with food significantly enhances its absorption (16). The starting dose for controlled release carbimazole is 15 mg administered once daily. In cats with mild hyperthyroidism (total T4 concentration <100 nmol/L or < 8 μg/dl), a 10 mg once daily is recommended (2-4,16).

Transdermal Antithyroid Drugs
Carbimazole and methimazole can be reformulated by a veterinary compounding pharmacy and applied to the non-haired inner portion of a cat’s pinnae (ear lobe) for transdermal administration (18-20). Such custom formulation increases expense of therapy and the stability of the product can never be guaranteed. To prevent absorption of the drug through one's own skin, it is best to wear gloves or a finger cot for application, and wash your hands afterwards.

Both antithyroid drugs are generally effective in cats when administered at a dose of 1.25 mg to 2.5 mg twice daily transdermally (2-4). One advantage of using a compounded formulation of methimazole (or carbimazole) over the 2.5- or 5-mg tablets is that it is easier to make smaller or finer dose adjustments.

Transdermal administration is associated with fewer gastrointestinal side effects than the oral route (19,21,22), but some cats resent manipulation of their ears and crusting can occur between doses leading to erythema. These problems can usually be prevented by removing any crusted material and cleaning the ear flap prior to administration.

Monitoring of Hyperthyroid Cats on Antithyroid Drug Treatment

Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily (1-4,21-23).

Initial Monitoring
Initially, cats should be reassessed after 2 to 3 weeks and a serum total T4 concentration measured. When monitoring, time of serum T4 sampling in relation to the administration of the antithyroid drug is not important (1-4,24). The goal of medical therapy is to maintain total T4 concentrations within the lower half of the reference range (1-4). Low serum T4 values should be avoided, however, because it has been shown that chronic hypothyroidism is deleterious to kidney function and may worsen already present chronic kidney disease (25,26).

If hyperthyroidism persists during antithyroid drug treatment, lack of owner or cat compliance should always first be eliminated as a reason for the failure of therapy. If the serum T4 concentrations remain high with proper treatment, however, the daily dose of methimazole or carbimazole can be increased in 2.5-mg increments, reassessing the cat again in 3 to 4 weeks (1-4).

Long-Term Monitoring and Treatment
For chronic management (once euthyroidism has been achieved), the daily antithyroid drug dosage is adjusted to the lowest possible dose that effectively maintains euthyroidism. Once the dosage has stabilized, the cat should be monitored every 3 to 6 months and as needed clinically. At time of each of these rechecks, a complete physical examination should be perform together with determination of a complete blood count, serum chemistry profile, and serum T4 concentration.

Relapses are common in cats treated with an antithyroid drug. Some cats will become more difficult to medicate over time, whereas others will need higher daily drug dosages to inhibit thyroid hormone secretion as their thyroid tumors continue to grow larger and larger (27).

It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L). Recent research indicates that hyperthyroidism may contribute to the development or progression of chronic renal disease in cats (28-30). Leaving a hyperthyroid cat untreated (or poorly regulated with methimazole or carbimazole) may therefore be detrimental to long-term kidney function and is never recommended.

During long-term treatment, it is again important to avoid inducing hypothyroidism, which may be deleterious to the cat’s kidney function (25,26). If hypothyroidism is suspected (which can develop even if the T4 is low-normal), a complete thyroid panel is recommended, including determination of the serum concentrations of total T4, free T4, T3, and TSH (see my previous blog posts on diagnostic testing for more information about these tests). The findings of low serum free T4 with high TSH concentrations is diagnostic for iatrogenic hypothyroidism; in those cats, the daily dose of methimazole should be decreased.

Because antithyroid medications have no effect on the underlying lesion, the thyroid nodules continue to grow larger and larger over time. This may necessitate an increased daily dose with time. In some cats, large enough dosages can no longer be administered to control the hyperthyroidism and surgery or radioiodine is needed to control the hyperthyroidism (27).

Side Effects & Adverse Reactions

Most clinical adverse reactions occur within the first 3 months of therapy (1-4). Mild clinical side effects of vomiting, anorexia, or depression occur in approximately 10-15% of cats, usually within the first 3 weeks of therapy (4,5). In most cats, these reactions are transient and do not require permanent drug withdrawal.

Mild Side Effects

Figure 1: Facial excoriations
due to methimazole

Early in the course of drug therapy, mild and transient hematological abnormalities, including leucopenia (low total white blood cell count), lymphocytosis (high lymphocyte count), or eosinophilia (high eosinophil count) develop in up to 15% of cats without any apparent clinical effect (1-6).

Self-induced excoriations of the head and neck (from scratching) occasionally develop, usually within the first 6 weeks of therapy (see Figure 1).

Less commonly, generalized enlargement of lymph nodes may develop during drug treatment (31). If either of these adverse effects occur, the drug must be stopped and another form of treatment given.

Life Threatening Side Effects

Figure 2: Bleeding from gums due
to methimazole

More serious hematological complications occur in less than 5% of cats and include a severe lowering of the white blood cell count (agranulocytosis) or platelet count (thrombocytopenia). Liver dysfunction, characterized by marked increases in all hepatic enzymes, develops in less than 2% of cats (1-5).

Rarely, cats may also develop a severe bleeding tendency during drug treatment (see Figure 2) (5,32). All of these adverse effects are reversible upon discontinuation of the medication.

The Bottom Line

Short-term or chronic management with antithyroid drugs provides a useful treatment option for many cats with hyperthyroidism. However, this treatment does not cure the disease and requires daily medication for the rest of the cat's life. Like any prescription drug, methimazole or carbimazole can produce adverse side effects, which may be life-threatening is some cats. For all of these reasons, antithyroid drugs will never be the "treatment of choice" for all cats with hyperthyroidism.

References

Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Barcelona, Multimedica, 2008; 127-168.
Baral R, Peterson ME. Thyroid diseases. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; in press.
Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157.
Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. Journal of Small Animal Practice 1992;33:228–235.
Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
Becker TJ, Graves TK, Kruger JM, et al. Effects of methimazole on renal function in cats with hyperthyroidism. Journal of the American Animal Hospital Association 2000;36:215–223.
Peterson ME, Hurvitz AI, Leib MS, Cavanagh PG, Dutton RE. Propylthiouracil-associated hemolytic anemia, thrombocytopenia, and antinuclear antibodies in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1984;184:806-808.
Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355.
Plumb DC. Plumb's Veterinary Drug Handbook (7th Ed). PharmaVet Inc, Stockholm, Wisconsin 2011.
Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy. Australian Veterinary Journal 2000;78:462-465.
Frénais R, Burgaud S, Horspool LJ. Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats. Journal of Veterinary Pharmacology and Therapeutics 2008;31:213-219.
Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. Journal of Small Animal Practice 2009;50:510-515.
Hoffman S, Yoder A, Trepanier L. Bioavailability of transdermal methimazole in a pluronic lecithin organogel (PLO) in healthy cats. Journal of Veterinary Pharmacology and Therapeutics 2002;25:189-193.
Hoffman SB, Marks SL, Taboada J et al. Transdermal methimazole treatment in cats with hyperthyroidism. Journal of Feline Medicine and Surgery 2003;5:77–82.
Buijtels JJ, Kurvers IA, Galac S et al. Transdermal carbimazole for the treatment of feline hyperthyroidism, Tijdschrift voor Diergeneeskunde 2006;131:478-482.
Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788.
Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28.
Trepanier LA, Hoffman SB, Knoll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 2003;222:954–958.
Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. Journal of Veterinary Internal Medicine 2009;23:1025-1030.
Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 2010;24:863-869.
Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092.
Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
Lapointe C, Bélanger MC, Dunn M, et al. N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2008;22:1103-1110.

van Hoek I, Lefebvre HP, Peremans K, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domestic Animal Endocrinology 2009;36:45-56.
van Hoek I, Meyer E, Duchateau L, et al. Retinol-binding protein in serum and urine of hyperthyroid cats before and after treatment with radioiodine. Journal of Veterinary Internal Medicine 2009;23:1031-1037.
Niessen SJ, Voyce MJ, de Villiers L, et al. Generalised lymphadenomegaly associated with methimazole treatment in a hyperthyroid cat. Journal of Small Animal Practice 2007;48:165-168.
Randolph JF, DeMarco J, Center SA, et al. Prothrombin, activated partial thromboplastin, and proteins induced by vitamin K absence or antagonists clotting times in 20 hyperthyroid cats before and after methimazole treatment. Journal of Veterinary Internal Medicine 2000;14:56-59.

Diet and Nutritional Management for Hyperthyroid Cats

2011-10-17T13:43:00.000-04:00

Proper nutrition plays an extremely important role in the treatment of a cat with hyperthyroidism. Hyperthyroid cats develop muscle wasting as well as many metabolic complications because of their disease. Therefore, they have special dietary needs and require sufficient amounts of all essential nutrients in their daily diets. This includes adequate amounts of high-quality proteins, fat, minerals, vitamins, and water.

The question, “What’s the best diet to feed my hyperthyroid cat?” is an extremely common one that I get from concerned cat owners. In this post, I will discuss the ideal food composition and nutrients that I believe hyperthyroid cats should be fed.

The Many Metabolic Problems Facing the Hyperthyroid Cat

When secreted in excess, thyroid hormones have profound metabolic effects on the whole body, and dysfunction of multiple organ systems is common in hyperthyroid cats (1-3).

Weight Loss and Muscle Wasting

Weight loss, despite a normal to increased appetite, is the classic and most common sign seen in cats with hyperthyroidism (1-3). These cats lose weight because their hyperthyroidism accelerates their metabolic rate and body’s energy expenditure. In other words, they are burning up their food calories faster than they can consume their daily meals.

It’s important to realize that hyperthyroidism is a catabolic wasting state, in which a "breaking down" of the body occurs no matter how much nutritional intake occurs. The progressive weight loss and muscle wasting that is so characteristic of feline disease is caused by an increased rate of fat and muscle protein breakdown (4,5).

When hyperthyroid cats initially lose weight, this can be first noticed as a loss of muscle mass over the cat’s lower back. Despite this loss of muscle mass, most mildly hyperthyroid cats retain their “belly” during the initial stages of their thyroid disease and may even have a higher than ideal body condition score.

With time, severe muscle wasting, emaciation, cachexia, and death from starvation can occur if the cat’s hyperthyroidism is left untreated (1-3). In hyperthyroidism, the cat’s body consumes its own muscle tissue to get the protein it needs to sustain its carnivorous life.

Even with treatment of hyperthyroidism, recovery of muscle mass and function may be prolonged, lasting several weeks to months. This is especially true if these cats are not provided with enough protein in their diet to rebuild and maintain their lost muscle mass.

High Blood Glucose, Insulin Resistance, and Diabetes Mellitus
Hyperthyroid cats can also develop profound changes in carbohydrate metabolism (glucose and insulin metabolism). Slightly high resting blood glucose (sugar) concentrations are common in hyperthyroid cats, which is generally attributed to a “stress” reaction.

However, the actual metabolic changes are actually much more complicated. Hyperthyroidism frequently causes moderate to severe insulin resistance (6,7), which is a physiological condition where the natural hormone insulin becomes less effective at lowering blood glucose levels. This insulin resistance is associated with a decreased glucose clearance, which is indicative of a prediabetic state. Occasionally, an untreated hyperthyroid cat will even go on to develop full-blown diabetes mellitus. Many of these diabetic cats are difficult to regulate with insulin therapy but treatment of their concurrent hyperthyroid state generally improves diabetic control.

Unfortunately, the insulin resistance and associated prediabetic state — so common in hyperthyroid cats —do not always improve despite successful treatment of hyperthyroidism (7). This indicates that hyperthyroid cats may have long-lasting alterations of carbohydrate metabolism that cannot always be reversed by treatment. In accord with that, some of these hyperthyroid cats (not diabetic at time of diagnosis) will go on to develop overt diabetes mellitus in the months to years after treatment of hyperthyroidism.

Sarcopenia of Aging
In addition to loss of muscle mass from the catabolic effects of thyroid hormone excess, cats also tend to lose muscle mass as they age, independent of their thyroid status. This phenomenon, referred to as sarcopenia of aging, is also common in elderly human beings (8-10). The term age-related sarcopenia is derived from Greek (meaning "poverty of flesh") and is characterized by a degenerative loss of skeletal muscle mass and strength, as well as increased muscle fatigability.

In adult cats, maintenance energy requirements decrease by about 3% per year until the age of 11 years, and then actually start to increase again (11). This contributes to a tendency of senior cats to lose muscle mass if their energy needs are not met. Lean body mass of aging cats drops dramatically after 12 years of age, and by age 15, cats may have a mean lean tissue mass that is a third less than cats aged 7 years or less (11, 12). Body fat also tends to progressively decrease in cats after the age of 12 years; this combination of reduced lean mass and body fat contributes to weight loss experienced by many elderly cats.

The ability to digest protein is also compromised in many geriatric cats. After the age of 14 years, one-fifth of geriatric cats have reduced ability to digest protein (11-13). Reduced protein digestibility in geriatric cats seems to occur in parallel with reduction of lean tissue and it might predispose them to negative nitrogen balance. (14).

Although moderation of calorie intake might be suitable for some mature cats, it does not appear to match the needs of most geriatric cats. In contrast, it seems more logical to use highly digestible, energy-dense food for geriatric cats in order to prevent or slow their decline in body weight and lean body tissue (11,14,15). Reducing protein intake in geriatric cats, at a time when lean tissue has been lost, is contraindicated. Geriatric cats seem to have nutritional requirements closer to kittens than to mature adult cat.

Diet Recommendations for Hyperthyroid Cats

Cats are true obligate carnivores (16-20). This means they must eat meat to survive; cats cannot be vegetarians. To me, it makes a great deal of sense to feed hyperthyroid cats, a diet with a composition close to what they would be getting in the wild. That would be a diet composed of approximately 50-60% protein, 5-10% carbohydrates, and 30-50% fat (21-23).

High Dietary Protein
As obligate carnivores, cats are unique in their need for large amounts of dietary protein (specifically, dispensable nitrogen) that separates them from omnivores and herbivore species (16-20). This absolute requirement for dietary protein intake in cats is critically important when formulating a diet for hyperthyroid cats, in which protein catabolism and muscle wasting is universally present.

Protein is the primary macronutrient responsible for maintenance of muscle mass. Restoring and preserving any remaining muscle tissue in cats treated for hyperthyroidism depends upon the cat consuming a diet with sufficient amounts of high-quality protein. In addition, this recommendation for higher amounts of dietary protein does not change once euthyroidism has been restored. The dogma that all older cats should be fed reduced energy “senior” diets must be questioned based on what is now known about the increasing energy requirements and nutritional needs of older cats (10,11).

In most geriatric cats, logic dictates the use of highly digestible, energy-dense foods to mitigate the decline in body weight and lean body tissue and help avoid protein:calorie malnutrition (10,14,15). Protein reduction for this geriatric life stage, at a time when lean tissue is being lost, is contraindicated. Geriatric cats seem to have nutritional requirements closer to kittens than to mature adult cats.

Low Dietary Carbohydrates
Since most of these cats also have subclinical diabetes —as evidenced by their mild hyperglycemia, glucose intolerance, and insulin resistance — feeding a low carbohydrate diet (<10% of total calories) also is strongly recommended. This is what most endocrinologists (myself included) also recommend in cats with diabetes mellitus (24-26).

Feeding a low carbohydrate diet will improve insulin sensitivity, reduce the need for exogenous insulin, and help stabilize glucose metabolism in these cats (24-26). This may prevent the development of overt diabetes and control long-term obesity in these cats after successful control of the hyperthyroidism.

Concurrent Kidney Disease in the Hyperthyroid Cat
Concurrent chronic kidney disease (CKD) is common in hyperthyroid cats, occurring in up to 30% of cases. Cats with advanced CKD — IRIS Stage 3 or 4 — may need lower amounts of dietary protein to lessen uremic episodes (27). However, at least in early to mid-stage renal disease, lowering of the serum phosphate concentration is much more important in management than dietary protein restriction, and this can be easily accomplished with phosphate binders without lowering the protein content of the diet (28,29). For an explanation of the IRIS system used by veterinarians to stage CKD in cats, see this link: http://www.iris-kidney.com.

It may seem impossible, but no studies have conclusively demonstrated that severe restriction of protein alone will prevent further deterioration of kidney function in cats (30). The major problem that I have with some of the prescription kidney diets is that they restrict protein to the point that some cats — especially those with concurrent hyperthyroidism —will continue to catabolize their own muscle mass despite adequate control of the thyroid condition.

Selecting a Commercially Prepared Diet for Your Hyperthyroid Cat

To me, it makes a great deal of sense to feed hyperthyroid cats a diet with a composition close to what they would be getting in the wild. To that end, I'm looking for a diet that is composed of approximately 50% protein and <10% carbohydrates (21-23). Because older cats also loss lean muscle mass in association with the “sarcopenia of aging,” this diet composition needs to be continued even after one treats the cat’s hyperthyroidism and restores euthyroidism.

Determine the Composition of the Diet
Check out this website (http://binkyspage.tripod.com/foodfaq.html), which gives you a breakdown of the composition of the various prescription and over-the-counter diets. This website uses metabolizable energy (ME) values to evaluate food composition— in other words, it gives you the percent calories that comes from the protein, fat, and carbohydrate fractions of the diet, and allows us to compare various diets without worrying about their different water levels (31).

It turns out that many of the over-the-counter diets have a better composition of protein and carbohydrates than you might have thought — even better than many of the more expensive prescription diets. Very few of my hyperthyroid cat patients require a prescription diet to fulfill their nutritional needs.

The composition of almost all dry food cat diets are much too high in carbohydrates and most are too low in protein content. That is why I believe it's best to limit the amount of dry food that is fed to cats, or even better, not feed dry food at all.

Examine the Ingredients of the Diet
Once we have selected a few diets with the required composition breakdown of carbohydrates, protein, and fat, we next have to look at the ingredient list (32). Not all of the proteins in cat foods are equal in quality. Remember that quality meat is the best ingredient in a food and that meat by-products are a close second. Some vegetable and grains are fine, but they may supply a less bioavailable form of protein for cats and should not be the primary source of dietary protein.

When deprived of protein, carnivores will continue to break down muscle tissue to create the energy they need. By feeding only high-quality protein diets, we will help restore the cat’s muscle mass and improve strength and agility.

Choosing A Pet Food Company
In addition to looking at the composition and ingredients, we have to ensure that all of the essential nutrients are present in the diet. Some of the cat foods marketed as "holistic or natural" may not actual be totally balanced or contain all of the essential amino acids that a cat needs. The best way to determine if diet is totally nutritious is to examine the diet's nutritional profile, which shows the levels or concentrations of all essential nutrients (amino acids, fat, mineral, and vitamin) in the pet food (33).

For cat owners, I recommend that they choose two or more pet food companies known to have a good track record and feed those foods. I also choose foods that carry an AAFCO feeding claim to be complete and balanced for an adult or senior cat. I would be very careful in choosing a smaller company as the primary supplier for your cat's food. Small pet food companies are less likely to have veterinary nutrition specialists on their staff, and therefore, their diets may not always balanced and could result in nutritional deficiencies.

In addition to rotating brands, I also like to feed a variety of different flavors. Why? I believe it's safer to rotate between brands because companies formulate their diets differently. It also helps to determine which brands and flavors and foods the cat prefers; since diet preferences may change over time, varying the food helps maintain a good appetite, especially as the cat ages.

Homemade Diets for Your Hyperthyroid Cat

Homemade diets can certainly be as good as any commercial diet if properly formulated and contain all of the essential nutrients needed for the older cat. The advantage of these diets is that you know exactly what ingredients they contain (34, 35). These homemade diets also allow for people to choose exactly what type of ingredient to include in the food (e.g., organic, kosher, or biodynamic ingredients).

The risk of homemade diets being improperly formulated, however, is very high unless the cat owner seeks out a veterinary nutritionist to help formulate a properly balanced diet (34, 35). I've included the links for some of the available nutritional consultation services below:

Veterinary Nutritional Consultations — PetDiets.com
DVM Consulting — BalanceIT.com
Dr. Susan Wynn, Georgia Veterinary Specialists — SusanWynn.com
Clinical Nutrition Program, University of Missouri
Veterinary Nutrition Service, University of Tennessee College of Veterinary Medicine

All of these services are operated by or have board-certified veterinary nutritionist on staff. Some of these sites may require your regular veterinarian to contact them for the information, whereas others will help you directly in formulating a balanced and complete diet. It is important to stress the need for a higher protein/lower carbohydrates diet for your hyperthyroid cat — again, that is an important factor that is not widely appreciated, even among veterinary nutritionists.

The use of uncooked meat, organs, and eggs in homemade cat food recipes can be a reason for particular concern, especially if essential safe-handling practices are not followed. Raw meat can be contaminated with microorganisms that have the potential to cause infection or food poisoning, thus posing a public health risk (34-36). Although feeding a raw diet may be the most "natural" way to provide feline nutrition and many strong arguments have been made about why raw is better (37-39), there is no clear scientific evidence that feeding raw meat has a nutritional advantage over feeding cooked meat. Given the potential health risks, the FDA does not advocate feeding raw meat, poultry, or seafood to pets (36).

If you do decide to feed a raw diet to your cat, whole raw diets for cats are available for purchase online (e.g., felinespride.com). These diets are handled and prepared carefully by the processor, frozen immediately to help prevent bacterial contamination, and kept frozen until they are delivered to your door.

If a diet is unsafe to eat or fails to provide all of the essential nutrients, a homemade cooked or raw diet provides the worst kind of nutrition. Unless one is highly dedicated and becomes knowledgeable to ensure that the homemade diet is both safe and balanced, it's generally best to stick with the commercial, canned diets, selecting them based on the criteria listed above.

The Bottom Line

Proper nutrition plays an extremely important supportive role in the management of a cat with hyperthyroidism, which needs to be maintained even after successful treatment of the cat's hyperthyroid state. These cats should be fed a balanced and complete diet, ideally containing relatively low amounts of carbohydrates and higher amounts of protein. This diet composition will help restore and preserve the muscle mass that has been lost as a result of increased muscle protein breakdown characteristic of hyperthyroidism.

If commercial canned food is fed, feed balanced food with healthy ingredients from more than one good company. If you decide to go with a homemade diet, get the recipe balanced and formulated by a veterinary nutritionist. And of course, continue to check in with your veterinarian for regular physical and biochemical examinations to help pick up any emerging problems that may develop as time goes on.

References

Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110.
Joseph RJ, Peterson ME. Review and comparison of neuromuscular and central nervous system manifestations of hyperthyroidism in cats and humans. Progress in Veterinary Neurology 1992;3:114-119.
Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Morrison WL, Gibson JN, Jung RT, Rennie MJ. Skeletal muscle and whole body protein turnover in thyroid disease. European Journal of Clinical Investigation 1988;18:62–68.
Riis AL, Jørgensen JO, Gjedde S, et al. Whole body and forearm substrate metabolism in hyperthyroidism: evidence of increased basal muscle protein breakdown. American Journal of Physiology: Endocrinology and Metabolism 2005; 288:E1067-1073.
Hoenig M, Ferguson DC. Impairment of glucose tolerance in hyperthyroid cats. Journal of Endocrinology 1989;121:249-251.
Hoenig M, Peterson ME, Ferguson DC. Glucose tolerance and insulin secretion in spontaneously hyperthyroid cats. Research in Veterinary Science 1992;53:338-341.
Short KR, Nair KS. Mechanisms of sarcopenia of aging. Journal of Endocrinological Investigation 1999;22(5 Suppl):95-105.
Fujita S, Volpi E. Nutrition and sarcopenia of ageing. Nutrition Research Reviews 2004;17:69-76.
Wolfe RR. Sarcopenia of aging: Implications of the age-related loss of lean body mass. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 12-17.
Little S: Evaluation of the senior cat with weight loss, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Perez-Camargo G: Cat nutrition: What is new in the old? Compendium for Continuing Education for the Practicing Veterinarian 2004;26 (Suppl 2A):5-10.
Patil AR, Cupp C, Pérez-Camargo G. Incidence of impaired nutrient digestibility in aging cats. Nestlé Purina Nutrition Forum Proceedings. 2003;26,2(A):72.
Wakshlag JJ. Dietary protein consumption in the healthy aging companion animal. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 32-39.
Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42.
MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562.
Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutrition Research Reviews 2002;15:153-168.
Zaghini G, Biagi G. Nutritional peculiarities and diet palatability in the cat. Veterinary Research Communications 2005;29, Suppl 2:39-44
Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association 2002;221:1559-1567.
Zoran DL, Buffington CA. Effects of nutrition choices and lifestyle changes on the well-being of cats, a carnivore that has moved indoors. Journal of the American Veterinary Medical Association 2011;239:596-606.
Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
Crissey SD, Slifka KA, Lintzenich BA. Whole body cholesterol, fat, and fatty acid concentrations of mice (Mus domesticus) used as a food source. Journal of Zoo and Wildlife Medicine 1999;30:222-227.
Rucinsky R, Cook A, Haley S, et al. AAHA diabetes management guidelines for dogs and cats. Journal of the American Animal Hospital Association 2010;46:215-224.
Frank G, Anderson W, Pazak H, et al. Use of a high-protein diet in the management of feline diabetes mellitus. Veterinary Therapeutics 2001;2:238-246.
Rand JS, Fleeman LM, Farrow HA, et al. Canine and feline diabetes mellitus: nature or nurture? The Journal of 2004;134(8 Suppl):2072S-2080S.
Plotnick A. Feline chronic renal failure: Long-term medical management. Compendium for Continuing Education for the Practicing Veterinarian 2007;29:342-324, 346-350.
Kidder AC, Chew D. Treatment options for hyperphosphatemia in feline CKD: what's out there? Journal of Feline Medicine and 2009;11:913-924.
Schmidt B, Spiecker-Hauser U, Murphy M. Efficacy and safety of Lantharenol on phosphorus metabolism in cats with chronic kidney disease. American College of Veterinary Internal Medicine Forum, 2008.
Ross SJ, Osborne CA, Kirk CA, et al. Clinical evaluation of dietary modification for treatment of spontaneous chronic kidney disease in cats. Journal of the Veterinary Medical Association 2006;229:949-957.
Laflamme DP. Determining metabolizable energy content in commercial pet foods. Journal of Animal Physiolology and Animal Nutrition (Berlin). 2001; 85:222-230.
Thompson A. Ingredients: where pet food starts. Topics in Companion Animal Medicine 2008;23:127-132.
FDA. Selecting nutritious pet foods.
Remillard RL. Homemade diets: attributes, pitfalls, and a call for action. Topics in Companion Animal Medicine 2008;23:137-142.
Schenck PA. Home-Prepared Dog and Cat Diets. Second Edition. Wiley-Blackwell, Ames, Iowa. 2010.
FDA. Is it safe for me to provide my pet with a raw food diet?
Hodgkins E. Feeding raw meat to your cat — is it safe and sensible? In: Your Cat: Simple New Secrets to a Longer, Stronger Life. Thomas Dunne Book, New York. 2007;181-188.
Feline Nutrition Education Society. Just what is raw feeding, anyway?
Pierson LA, CatInfo.org. Making cat food.

Daily Water and Fluid Requirements and Needs for Hyperthyroid Cats

2011-10-10T22:59:00.001-04:00

Hyperthyroid cat drinking water
in his condo at the Hypurrcat spa

Drinking a healthy amount of water is vital to a cat’s health. Although water is certainly important for clinically normal cats, it is absolutely essential in maintaining and restoring health in sick cats, and that certainly includes cats with hyperthyroidism.

Everything your cat does uses energy, and water plays a critical role in the body's metabolic processes, which regulate all the body's functions (1,2). This is especially true in hyperthyroid cats, in which the metabolic rate is increased, body temperature tends to be elevated, and increased moisture loss through the respiratory and gastrointestinal routes are common.

Many hyperthyroid cats are mildly to moderately dehydrated. Keeping a hyperthyroid cat well hydrated helps ensure that adequate oxygen and vital nutrients will reach all the tissues of the body — this allows the cat to metabolize its nutrients and sustain or restore normal body function. Maintaining adequate hydration also helps absorb the excess body heat typically generated by hyperthyroid cats because of their “revved up” metabolic state (3-5).

The Potential for Dehydration is High in Older, Geriatric Cats

As cats age, they develop a number of important changes in water metabolism that can predispose them to dehydration. First of all, even healthy geriatric cats have higher water losses than younger cats, possibly due to reduced urine concentrating ability even without obvious evidence of overt kidney disease (6,7). In addition, the poor thirst reflect, already present in the younger cat (1,7,8), worsens as the cat ages; again, this reduced sensitivity to thirst commonly leads to a state of chronic dehydration in the geriatric cat.

The potential for dehydration will be exacerbated in cats with concurrent diseases that cause increased thirst (polydipsia) and urination (polyuria), such as diabetes mellitus, chronic kidney disease, or hyperthyroidism. Senior cats may benefit from increased water content via feeding canned foods, ensuring access to fresh water, or flavoring the water (see Water and Hydration Recommendations for Hyperthyroid Cats, below). Older cats do not cope well with changes in daily routine, so any changes to food and water should be made gradually.

Hyperthyroid Cats Commonly Have Increased Thirst and Urination

Thyroid hormones have a diuretic action, an effect that was reported in cats almost 70 years ago (9). In accord with those experimental findings, increased thirst (polydipsia) and urination (polyuria) are frequent clinical signs observed in hyperthyroidism, occurring in over half of affected cats (3-5). Because of that effect, hyperthyroidism (along with kidney disease and diabetes) is one of the three major rule-outs for a cat presenting with increased thirst and urination. Although concurrent primary kidney disease contributes to polyuria and polydipsia in up to a third of cats with hyperthyroidism (5), these signs also occur in many cats without any evidence for kidney disease.

The precise cause of these signs in hyperthyroid cats without any kidney disease is not always clear. However, the hyperthyroid state may impair urine concentrating ability by increasing total blood flow to the kidneys, thereby decreasing the solute concentrations in the inner part of the kidney (i.e., the renal medulla). This renal “medullary washout” may cause polyuria with secondary polydipsia (5, 10). In other words, these cats drink more to compensate for the increased amounts of water lost through their kidneys.

Alternately, in cats with normal renal concentrating ability, a central nervous system disturbance caused by hyperthyroidism may produce a “compulsive” primary polydipsia (3-5,10,11). In other words, these cats develop a compulsive need to drink more water, and the increased urination occurs as a secondary response to the large volumes of water consumed. In these hyperthyroid cats, increased thirst and urination will normalize within a few weeks after successful of treatment of hyperthyroidism.

In addition, it has been suggested that diseases that promote polyuria and a dilute urine (such as hyperthyroidism, diabetes, or renal disease) predispose to urinary tract infections (12,13). In one study, over 20 percent of cats with untreated hyperthyroidism has a positive urine culture, diagnostic for a urinary tract infection (13). If this infection would spread to the kidneys, pyelonephritis and subsequent renal failure could result (10).

As we discussed in my last post on Daily Water Requirements and Needs for Cats, cats have failed to evolve a strong "thirst reflex" like that of dogs and humans. This inherited weakness of the cat's thirst drive to respond to changes in his or her state of hydration must be considered in the hyperthyroid cat. We must work to ensure that an adequate hydration status is maintained on a daily basis in these cats.

Water and Hydration Recommendations for Hyperthyroid Cats

An important aging change in cats is reduced sensitivity to thirst, resulting in an increased risk of dehydration even in cats with seemingly normal renal function. As animals go, cats require less water than many others, and we often have a difficult time getting cats to drink as much as we would like. Cats with certain health problems, especially those with hyperthyroidism, need to drink more water than an average cat.

But how can we get a hyperthyroid cat to drink more water?
Here are a few hints:

Place more bowls of fresh water located in different areas of the house —Providing cats with more than one bowl of water in different locations around the house or apartment will encourage the cat to stop and take a sip from each one. I normally recommend at least to three bowls in various locations throughout a cat’s living space.
Provide larger water bowls —Another way to encourage cats to drink more water is to provide larger water bowls, such as ones designed for dogs, to prevent the cat`s whiskers from touching the sides when drinking. Filling the water level all the way to the top also helps prevent the whiskers from touching the sides of the bowl. A cat’s whiskers are very sensitive; whiskers touching the sides of the bowl can be very irritating for the cats.
Keep water bowls clean— By nature, cat will not drink stagnant or dirty water. Standing water tends to get warm and stale and can harbor bacteria, dust, and insects. To prevent this problem, it’s important to wash the bowl daily and provide fresh, clean water. In addition, use of stainless steel or ceramic bowls is best, as plastic bowls may lock in bacteria and odors.
Provide running water— Running water is naturally enticing to some cats. Some cats that refuse to drink out of a water bowl will drink water from a slowly running faucet or a “fountain bowl,” which can provide your cat with running water 24 hours a day. Make sure that the cats are drinking out of the fountain before removing other water bowls.
Flavor the water or add ice cubes— Adding a little water from a can of water-packed tuna to the bowl of water provides extra flavor to the water, which may entice some cats to drink. Flavoring water with chicken broth is another useful means to convince cats to drink more. Adding ice cubes or chips to the water as a treat will also help in some cats.
Feed canned food— Canned food has a lot of moisture, so it will provide your cat with much more water than feeding dry food. See my last post on Daily Water Requirements and Needs for Cats for more information about how to calculate the amount of water that the food contains.
Add water to the food— In some cats, 3 to 4 teaspoons of warm water can be added to the canned or dry food to provide more water. However, some cats do not like food with added water and may refuse to eat it, so this approach must be individualized.
Filtered or spring water — Some cats prefer filtered or spring water over tap water. However, distilled water is never a good choice, as it may actually flush needed minerals out of the cat’s body. I do not advocate the use of bottled water because of the environmental consequences of the plastic bottles as waste (14), as well as the chemical plasticizers known to leach from the plastic bottles. In addition, it’s important to realize that there is no way to know if the bottled water you buy is really pure and natural, or if it’s just processed and packaged tap water.

Bottom Line: Water and Hyperthyroid Cats

Since up to a third of hyperthyroid cats have concurrent kidney disease, and up to 20 percent have concurrent urinary tract infections, keeping our hyperthyroid cats well hydrated is of utmost importance. Feeding a canned diet containing 70 to 80 percent moisture together with plenty of fresh water freely available helps guarantee control of water balance in both normal cats and cats with hyperthyroidism.

Again, water serves as the hub of all chemical processes in the body. Ensuring and maintaining proper hydration plays a key role in treatment of all cats with hyperthyroidism.

References

Case LP. Nutrition: feeding cats for health and longevity. In: The Cat: Its Behavior, Nutrition and Health. Iowa State Press, Ames, Iowa 2003;289-327.
Kohn CW, DiBartola SP. Composition and distribution of body fluids in dogs and cats. In: DiBartola SP, ed. Fluid therapy in small animal practice. Philadelphia: WB Saunders Co, 1992;1–34.
Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110.
Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995;206:302-305.
Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
Pérez-Camargo G. Feline decline in key physiological reserves: implication for mortality. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology. St. Louis, MO. 2010, pp. 6-13.
Little S. Managing the senior cat. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
National Research Council. Feeding behavior of dogs and cats. In: Nutritional Requirements of Dogs and Cats. National Academies Press, Washington, D.C. 2006; pp 22-27.
Radcliffe CE: Observations on the relationship of the thyroid to the polyuria of experimental diabetes insipidus. Endocrinology 1943;32:415-421.
Nichols R, Peterson ME: Investigation of polyuria and polydipsia, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, in press.
Evered DC, Hayter CJ, Surveyor I. Primary polydipsia in thyrotoxicosis. Metabolism 1972;21:393-404.
Mayer-Roenne B, Goldstein RE, Erb HN: Urinary tract infections in cats with hyperthyroidism, diabetes mellitus and chronic kidney disease. J Feline Med Surg 2007;9:124-132.
Bailiff NT, Westropp JL, Nelosn RW, et al. Evaluation of urine specific gravity and urine sediment as risk factors for urinary tract infections in cats. Veterinary Clinical Pathology 2008;37:317–322.
Is Bottled Water Better? Medicine.Net.com

Daily Water Requirements and Needs for Cats

2011-10-06T09:38:00.000-04:00

Running water is naturally enticing
to many cats and is one way to get
cats to drink more water

Drinking a healthy amount of water is vital to a cat’s health. Most people don’t think of water as a nutrient. But considering that water accounts for about two-thirds of a cat’s body weight and serves as the hub of all chemical processes in the body, it's actually the king of all nutrients (1,2).

Water serves many physiological functions: it transports nutrients and oxygen through the blood stream and into the cells, moisturizes the air in the lungs, regulates body temperature, protects and moisturizes the joints and internal organs, and helps eliminate waste products of metabolism through the kidneys and gastrointestinal tract.

Cats Do Not Naturally Drink Much Water

Cats do not normally need to drink very much water. Cats have evolved to obtain their water requirements almost entirely on the moisture content in their food. In the wild, cats obtain most of their water from freshly killed prey (e.g., small rodents, birds, amphibians, and insects), all of which contain about 70 to 75 percent water (3-7). Cats can do well for long periods without drinking any water when receiving canned food containing 67 to 73 percent water (8, 9); however, they will become dehydrated when the water content of the food drops to less than 61 percent (9). Therefore, normal cats eating rodents or birds or house cats eating canned-only foods may obtain enough water in their diets so that extra drinking water may not always be needed (2,10).

It's very important to realize that domestic cats have a diminished "thirst drive" and ideally will derive most of their daily water intake from the moisture contained in their food. In this respect, control of water balance in cats differs markedly from that of dogs and most other animals. When dehydrated, cats are slower to initiate drinking or to drink enough for complete rehydration — one study found that dehydrated dogs will drink enough to replenish 6 percent of their body weight in an hour compared to the 24 hours it takes for dehydrated cats (11).

In response to changes in the water content of food, cats adjust their voluntary water intake less precisely and less rapidly than do dogs. Similarly, their compensatory drinking response to dehydration induced by higher environmental temperatures or concurrent disease is less effective than it is in dogs.

How Much Water Does a Cat Need to Drink?

The amount of water drunk by an individual cat depends on a variety of factors, including the cat's size and activity, the season, and whether the cat's diet includes wet food or dry cat food only (2,10,12). Factors such as high heat, exercise, or lactation can double or triple the amount a cat drinks. And, of course, diseases such as hyperthyroidism, kidney disease, and diabetes will all greatly increase a cat's daily water requirements (I'll talk more about water and hydration in these cats in my next post).

So how much should a cat drink? A normal cat’s daily water requirement ranges from 5 to 10 fluid ounces per day (or an average of 60 ml/kg/day). Cats eating canned food will receive much of their daily water needs from its food, since canned food is about 70 to 80 percent water. In contrast, dry food is only 7 to 10 percent water. Normal cats eating canned food may need to drink less than 1 ounce of additional water per day, whereas a cat consuming only a dry diet may need to drink over 7 ounces per day to stay hydrated. This higher amount of water can be difficult to achieve because cats are not prone by nature to drink large amounts of water.

Will Type of Food (Dry vs. Canned) Affect the Amount of Water a Cat Drinks?

A cat consuming a predominantly dry food diet will drink more water than a cat consuming a canned food diet. But in the end, when water from all sources is added together (moisture in their diet plus the water they drink), the cat on dry food consumes about half the amount of water required for adequate hydration compared to a cat eating canned food (2, 13).

Put another way, when cats are fed only dry food, they do increase the amount of water drunk but not nearly enough to fully compensate. In one study (12), cats consuming a dry food diet containing 10% moisture with free access to drinking water had an average daily urine volume of 60 milliliters (or 2 fluid ounces). This urine volume almost doubled when the cats were then fed a canned diet containing 75 percent moisture.

Calculating the Volume of Water Provided by the Cat’s Diet

The moisture content of all cat food (both canned and dry) is listed on the product label as part of the Guaranteed Analysis (14). Canned diets usually contain about 75% moisture, so for every 100 grams of food fed, 75 grams (which equals 75 ml) is water. Dry foods, on the other hand, contain only about 10% moisture, so for every 100 grams of food fed, 10 ml if water.

The weight of a standard, larger can of cat food is 5.5 ounces (156 grams). If the moisture content of the food is listed as 78 percent max and the cat eats the entire can of food, he or she would be ingesting approximately 120 ml of water from the food (156 gm X 0.78 = 121.68 ml). Again, if the same cat at 156 grams of a dry food containing 10% moisture, that would provide only 15.6 ml of water.

The weight of a standard, smaller can of cat food is 3 ounces (85 grams). If the moisture content of the food is again listed as 78 percent max and the cat eats the entire can of food, he or she would be ingesting approximately 65 ml of water from the food (85 gm X 0.78 = 66.3 ml). Again, if the same cat ate 85 grams of a dry food containing 10% moisture, that would provide only 8.5 ml of water.

Water Requirements in Cats with Hyperthyroidism

Although water is certainly important for clinically normal cats, it is absolutely essential in maintaining and restoring health in sick cats, and that certainly includes cats with hyperthyroidism. To that end, I'll address the special water needs of hyperthyroid cats in my next post.

References

Kohn CW, DiBartola SP. Composition and distribution of body fluids in dogs and cats. In: DiBartola SP, ed. Fluid therapy in small animal practice. Philadelphia: WB Saunders Co, 1992;1–34.
Case LP. Nutrition: feeding cats for health and longevity. In: The Cat: Its Behavior, Nutrition and Health. Iowa State Press, Ames, IA 2003;289-327.
Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
Mutze GJ, Green B, Newgrain K. Water flux and energy use in wild house mice and the impact of seasonal aridity on breeding and population levels. Oecologia 1991;88:529–538. 12.
Angilletta MJ. Estimating body composition of lizards from total body electrical conductivity and total body water. Copeia 1999;3:587–595.
Austad S, Kristan D. Does caloric restriction of laboratory mice mimic natural food intake of wild mice? Gerontology 2002;42:8–18.
Hall JM, Hung F, Zurich MW. The influence of diet on the body condition of the house cricket and consequences for their use in zoo animal nutrition. Zoologische Garten 2003;73:238–244.
Caldwell FT. Studies in water metabolism of the cat. The influence of dehydration on blood concentration, thermoregulation, respiratory exchange, and metabolic-water production. Physiological Zoology 1931; 4:324-355.
Prentiss PG. Wolf AV. Eddy HE. Hypopenia in cat and dog: ability of the cat to meet its water requirements solely from a diet of fish or meat. American Journal of Physiology 1959; 196:625-632.
National Research Council. Feeding behavior of dogs and cats. In: Nutritional Requirements of Dogs and Cats. National Academies Press, Washington, D.C. 2006; pp 22-27
Adolph EF. Tolerance to heat and dehydration in several species of mammals. American Journal of Physiology 1947;151:564–575.
Seefeldt SL, Chapman TE. Body water content and turnover in cats fed dry and canned rations. Am J Vet Res 1979;40:183–185.
Kirk CA, Debraekeleer J, Armstrong PJ. Normal cats. In: Hand MS, Thatcher CD, Remillard RL, et al, eds. Small animal clinical nutrition. 4th ed. Philadelphia: WB Saunders Co, 2000;291–351.
Food and Drug Administration website. Pet food labels—general.

Treatment Options and Considerations for Hyperthyroid Cats

2011-09-25T22:29:00.000-04:00

The underlying cause of the thyroid tumors (adenomatous hyperplasia, adenoma, or carcinoma) associated with feline hyperthyroidism is not known. Because spontaneous remission of hyperthyroidism in cats does not occur, the aim of treatment is to control the excessive secretion of thyroid hormone from the adenomatous thyroid gland.

The 4 Treatment Options

In cats, hyperthyroidism can be treated in four ways — surgical thyroidectomy, radioactive iodine (131-I), chronic administration of an antithyroid drug, or lifelong feeding of an ultra-low iodine diet. Each form of treatment has advantages and disadvantages, which must be considered when formulating the best treatment plan for the individual hyperthyroid cat.

Formulating the Best Form of Treatment for Each Cat

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated cardiovascular diseases or other major medical problems (e.g., chronic kidney disease), availability of a skilled surgeon or radioiodine treatment facility, and owner's preference.

The cost of therapy is a major consideration for many cat owners. Medical treatment or use of low-iodine dietary therapy costs far less initially. However, the cost of ongoing monitoring can exceed that of thyroidectomy or radioiodine therapy over a period of many months to years.

Cure versus Control of Hyperthyroidism

Of the three forms of treatment available, it must be emphasized that only surgery and radioactive iodine remove and destroy the thyroid tumors, respectively, and thereby "cure" the hyperthyroid state.

Use of an antithyroid drug (e.g., methimazole or carbimazole) will block thyroid hormone synthesis. And chronic feeding of an iodine deficient diet will reduce the production of T4 and T3 by the thyroid tumor. However, because antithyroid drugs and low-iodine diets do not destroy the hyperthyroid cat’s thyroid tumor(s), hyperthyroidism will recur once antithyroid drugs or dietary therapy is discontinued.

Outline of Treatment

Over the next few weeks, I’ll be discussing each of the four treatment options for hyperthyroidism in some detail. In my next post, however, l will review a frequently neglected but very important part of any of the treatment options — that is, providing the hyperthyroid cat with proper nutrition and adequate amounts of water.

Hyperthyroid cats develop many muscle wasting as well as many metabolic complications because of their disease. Therefore, they have special dietary needs and require a high-quality nutrients in their daily diet including adequate amounts of water, protein, fat, vitamins, and minerals.

References

Birchard SJ, Peterson ME, Jacobson A: Surgical treatment of feline hyperthyroidism: Results of 85 cases. Journal of the American Animal Hospital Association 1984;20:705-709.
Peterson ME, Kintzer PP, Hurvitz AI: Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150-157.
Welches CD, Scavelli TD, Matthiesen DT, Peterson ME: Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Veterinary Surgery 1989;18:392-396.
Peterson ME, Becker DV: Radioiodine treatment of 524 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1995;207:1422-1428.
Peterson ME: Radioiodine treatment for hyperthyroidism. Clinical Techniques in Small Animal Practice 21:34-39, 2006
Peterson ME: Hyperthyroidism, In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Fifth Edition). Philadelphia, WB Saunders Co. 2000; pp 1400-1419.
Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008, pp 127-168.
Peterson ME: Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC (eds): Current Veterinary Therapy XIIII. Philadelphia, Saunders Elsevier, 2009, pp 180-184.
Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press
Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012; in press.
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.

Hyperthyroid in Cats: Table of Contents

2011-09-16T00:04:00.000-04:00

As you know if you have been following this blog, I've spent much of the last few months writing about issue of hyperthyroidism, the most common endocrine disorder of the cat.

Before I move on to the topics of treatment of this common feline condition, I thought I'd post links to the topics I've covered thus far:

I'm halfway through with this "book" on feline hyperthyroidism!

Confirming the Diagnosis of Hyperthyroidism in Cats: Thyroid Scintigraphy

2011-09-10T21:46:00.000-04:00

Dr. Mark Peterson performing a thyroid scan on one of his hyperthyroid cat patients

Thyroid scintigraphy (thyroid scanning or thyroid imaging) provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the diagnosis, staging, and management of thyroid disease in cats with suspected hyperthyroidism.

Thyroid scintigraphy is considered the “gold standard” for diagnosing mild hyperthyroidism in cats (1-3, 6,7). It is considered to be the most sensitive diagnostic test available — yes, even better than determination of serum T4, free T4, or TSH for diagnosis of hyperthyroidism.

In addition, thyroid scanning is an invaluable tool for evaluating the stage and extent of thyroid tumors (adenomas and carcinomas) in hyperthyroid cats (1,4,5,8).

How Thyroid Scintigraphy Works

To perform thyroid scintigraphy, the specialist injects the cat with a small dose of a radioactive tracer subcutaneously. Over the next hour, the cat’s salivary glands and thyroid glands take up the tracer. The radioactive tracer, now in the salivary and thyroid glands, then emits gamma rays (a high energy electromagnetic wave, a bit stronger than an X-ray), which are detected by a gamma camera to form an image.

To perform thyroid imaging, the cats sits normally on the camera (ventral view) or lies on his or her side (lateral view) while the gamma camera acquires the thyroid image (Figure1, above). The scanning process itself generally takes less than a minute and does not require sedation.

To watch video of Dr. Peterson performing a thyroid scan on a cat with hyperthyroidism, click on this link.

Use of Thyroid Scintigraphy as a Diagnostic Test

In normal cats, the thyroid gland appears on thyroid scans as two well-defined, focal (ovoid) areas of radionuclide accumulation in the cranial to middle cervical region. The two thyroid lobes are symmetrical in size and shape and are located side by side (Figure 2).

Figure 2: Thyroid scan of a normal cat.
Notice the similar uptake of the radionuclide in the thyroid lobes and the salivary glands.

The normal feline thyroid gland will take up about as much of the tracer as the salivary glands do (see Figure 2). On the scan, we expect the thyroid and salivary glands to be equally bright (a 1:1 brightness ratio). In addition to visual inspection, we can actually calculate the percent thyroidal uptake of the radioactive tracer as well as the exact the thyroid:salivary ratio. Both of these calculations are strongly correlated with circulating thyroid hormone concentrations and provide a extremely sensitive means of diagnosing hyperthyroidism (6,7).

Figure 3: Thyroid scan of a hyperthyroid cat with a single thyroid adenoma.
Notice the increased uptake of the radionuclide in the thyroid tumor compared with the salivary glands.

Figure 4: Thyroid scan of a hyperthyroid cat with bilateral thyroid adenomas.
Notice the increased uptake of the radionuclide in both thyroid tumors compared with the salivary glands.

In hyperthyroid cats, thyroid scintigraphy directly visualizes functional thyroid tissue (Figures 3 and 4). Based up the calculated percent tracer uptake or thyroid:salivary ratio, thyroid imaging can diagnose hyperthyroidism before laboratory tests are consistently abnormal. Thyroid scanning can also prevent misdiagnosis of hyperthyroidism in cats with falsely high serum thyroid hormone values.

More Uses for Thyroid Scintigraphy

Thyroid scintigraphy has four more uses in hyperthyroid cats, other than as a diagnostic test. First, it is an excellent method for evaluating the size of ectopic thyroid tissue, which can be located anywhere from base of the tongue to the heart (Figure 5). Second, it can locate large tumors that gravity has pulled into the thoracic cavity. Finally, thyroid scintigraphy also provides valuable information for diagnosing and evaluating cats with thyroid carcinomas (Figure 6).

Figure 5: Thyroid scan of a hyperthyroid cat with ectopic thyroid adenoma located with the chest cavity. Because of its location, this tumor could not be palpated on physical examination.

Figure 6: Thyroid scan of a hyperthyroid cat with a thyroid carcinoma (cancer). Note the extension of tumor beyond the limits of the normal thyroid capsule. This represents regional metastasis characteristic of carcinoma.

Once we locate a hyperthyroid cat's thyroid tumors on the scan, we can then measure the tumor size and calculate tumor volume (3). This is very helpful in individualizing the cat's dose of radioiodine.

Why Isn't Thyroid Scintigraphy Used More Often?

Apart from expense and the licensing needed to handle and administer radioisotopes, few veterinarians have access to the nuclear medicine equipment needed to obtain thyroid images or perform thyroid uptake determinations. Only a few large referral hospitals in the country offer nuclear scintigraphy to do thyroid scanning.

At the Animal Endocrine Clinic, we routinely perform thyroid scintigraphy on all of our cats in which the diagnosis is not completely clear cut (go to my website for more information). We believe that thyroid scintigraphy plays an essential role in the diagnosis and management of cats with hyperthyroidism.

References:

Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:10-16.
Daniel GB, Sharp DS, Nieckarz JA, et al. Quantitative thyroid scintigraphy as a predictor of serum thyroxin concentration in normal and hyperthyroid cats. Veterinary Radiology & Ultrasound 2002;43:374-382.
Forrest LJ, Baty CJ, Metcalf M.R, et al,Feline hyperthyroidism: Efficacy of treatment using volumetric analysis for radioiodine dose calculation. Veterinary Radiology & Ultrasound, 1996;37:141-145.
Harvey AM, Hibbert A, Barrett EL, et al. Scintigraphic findings in 120 hyperthyroid cats. Journal of Feline Medicine and Surgery 2009;11:96-106.
Kintzer PP, Peterson ME. Nuclear medicine of the thyroid gland. Scintigraphy and radioiodine therapy. Veterinary Clinics of North America: Small Animal Practice 1994;24:587-605.
Mooney CT, Thoday KL, Nicoll JJ, et al. Qualitative and quantitative thyroid imaging in feline hyperthyroidism using technetium-99m as pertechnetate. Veterinary Radiology & Ultrasound 1992;33,313-320.
Nap AM, Pollak YW, van den Brom WE, et al. Quantitative aspects of thyroid scintigraphy with pertechnetate (99m TcO4) in cats. Journal of Veterinary Internal Medicine 1994;8:302-303.
Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Veterinary Radiology 1984;25:23-27.

Confirming the Diagnosis of Hyperthyroidism: Thyrotropin-Releasing Hormone (TRH) Stimulation Test

2011-09-05T12:06:00.000-04:00

In my last blog post, I discussed use of dynamic test, the T3 suppression test. Suppression testing can be helpful in confirming the diagnosis when we suspect that a cat has mild hyperthyroidism but serum concentrations of total and free T4 are either normal or borderline.

An alternative dynamic test that can be used to help diagnose cats with mild or occult hyperthyroidism is the thyrotropin-releasing hormone (TRH) stimulation test (1-3). Compared with the T3 suppression test, this test takes the opposite approach — now we are stimulating, rather than suppressing, the secretion of TSH from the pituitary, which in turn, would lead to increased T4 secretion.

Thyrotropin-Releasing Hormone (TSH) Stimulation Test

TRH is the hormone released by the hypothalamus that subsequently stimulates the release of thyrotropin (TSH) from the pituitary gland (4). When administered intravenously to normal cats, TRH causes an prompt increase in TSH secretion and serum T4 concentrations (see Figure 1, left panel).

Figure 1: TRH tests in normal cats (left) & cats with hyperthyroidism (right)

In contrast to the situation in normal cats, administration of large doses of TRH to hyperthyroid cats has little or no effect on TSH or T4 secretion. The reason for this that pituitary TSH secretion has already been completely and chronically suppressed by the higher-than-normal thyroid hormone secretion characteristic of hyperthyroidism (see Figure 1 above, right panel).

Administration of only a single dose of TRH is not enough to stimulate chronically suppressed TSH-secreting cells.

Testing Protocol for the TRH Stimulation Test

To perform the TRH stimulation test in cats, the following protocol is recommended:

The veterinarian draws a blood sample for determination of baseline serum concentrations of total T4 (and T3).
TRH (Protirelin) is administered to the cat at the dosage of 100 μg/kg, IV.
Four hours later, the veterinarian again draws a blood sample for serum T4 (and T3) determinations.

One may ask: but what about measuring TSH? Indeed, that would be a more direct measure and would shorten the testing time from 4 hours to less than an hour (4). However, because use of TSH measurements in cats with thyroid disease have only recently been advocated, studies looking the serum TSH response to TRH have yet to be published in cats.

Interpretation of Results of TRH Stimulation Testing

When the TRH stimulation test is performed in normal cats and sick cats without hyperthyroidism (1), there is a consistent rise in serum T4 concentrations (approximately 2-fold rise). In contrast, when the test is performed in cats with hyperthyroidism, even in cats with only slightly high or high-normal resting serum T4 concentrations, there is little, if any, rise in serum T4 values after the administration of TRH (see Figure 2, below).

Figure 2 — Box plots of the serum T4 concentrations before (A) and after (B) TRH stimulation in 31 clinically normal cats, 35 cats with hyperthyroidism, and 15 cats with nonthyroidal disease. The "box" represents the interquartile range from the 25th to 75th percentile (represents the middle one-half of the data). The horizontal bar through the box is the median. The "whiskers" represent the main body of data, which in most cases is equal to the range. Outlying data points are represented by open circles. To convert serum T4 concentrations from nmol/L to µg/dl, divide the given values by 12.87.

Regarding interpretation of the TRH stimulation test results, we found that the relative rise (per cent increase) in serum T4 concentration after administration of TRH was the best (most sensitive) criteria for predicting whether or not cats are hyperthyroid (1). A percent rise in serum T4 of less than 50 per cent is consistent with mild hyperthyroidism, whereas a value of greater than 60 per cent is seen in normal cats; values between 50 and 60 per cent are equivocal or borderline responses.

The serum T3 response to TRH is less helpful in separating normal from hyperthyroid cats, because many normal cats have only a small and inconsistent rise in serum T3 concentrations after TRH administration. Therefore, I do not recommend determining the serum T3 response as part of the TRH stimulation test.

In another study of 36 cats with severe critical illness (5), half of the severely sick cats showed an inadequate T4 response to TRH stimulation test (i.e., a percent rise in serum T4 of less than 50 per cent). Unfortunately, this could create a situation where mild hyperthyroidism is falsely diagnosed in a cat with critical illness. Although the reason for this is unclear, critical nonthyroidal illness may render pituitary TSH secretion unresponsive to the stimulatory effects of TRH administration.

The bottom line: This test is not meant to differentiate normal and hyperthyroid cats from cats with severe critical illness. I do not recommend doing this test in severely ill cats, inasmuch as the test results may be invalid.

Which is Better: TRH Stimulation Test vs. the T3 Suppression Test?

Advantages of the TRH stimulation test over the T3 suppression test include the shorter time needed to perform the test (4 hours vs. 3 days), and the fact that the TRH stimulation test is not dependent upon the owner's ability to administer oral medication.

The major disadvantage of the TRH stimulation test in cats is that side effects almost invariably occur immediately after administration of the TRH (1, 6-8). These adverse effect include the following:

Excessive salivation (see Figure 3)
Vomiting
Induced defecation
Rapid breathing (tachypnea); open mouth breathing

Fortunately, all of the adverse side effects are transient and completely resolve by the end of the 4-hour test period. Because of these adverse effects, however, I generally do a T3 suppression test over the TRH stimulation test for dynamic testing of the pituitary-thyroid axis.

Figure 3: Cat showing increased salivation after TRH administration

Our studies show a close relationship between the presence (or absence) of suppressed serum T4 concentrations in response to T3 suppression and stimulated T4 values in response to TRH stimulation. Therefore, although the two tests evaluate the pituitary-thyroid axis in different ways, our findings indicate that the two screening tests provide similar information and can probably be used interchangeably for diagnosing mild hyperthyroidism in cats.

Future Investigations: Serum TSH Response to TRH Stimulation in Cats

In human beings, the TRH stimulation test is generally done by directly evaluating the change in serum TSH concentrations, not indirectly by measuring serum T4 values. When this is done, the TRH stimulation test is one of the most sensitive tests in detecting early or mild hyperthyroidism in human patients. Again, because TSH production is completely suppressed in hyperthyroid patients, they will not show a rise in serum TSH when TRH is administered, whereas normal individuals will respond with a brisk rise in circulating TSH within 30 minutes of TRH injection. The new, ultra-sensitive human TSH assays have largely eliminated the need for TRH stimulation testing, but it still remains a useful and time-proven test.

Similar serum TSH responses to TRH have been reported in normal dogs — baseline serum TSH concentrations increase 2- to 3-fold when measured 30 minutes after administration of TRH at doses ranging from 1-10 μg/kg. There are two major advantages of evaluating the TSH response rather than the T4 response to TRH:

The test period is shorten from 4 hours to only 15-30 minutes.
A much lower dose of TRH is needed, greatly reducing or eliminating the adverse signs commonly seen with the higher doses needed to elicit a serum T4 response.

Measuring the serum TSH response to TRH has not been reported in cats with hyperthyroidism. Obviously, this is an area that needs to be investigated, now that we know that the canine TSH assay can be used to measure circulating TSH in cats.

Because the canine TSH assay is not designed to measure low values, basal TSH measurements are not very useful as a diagnostic test in feline hyperthyroidism (see my previous blog on TSH in cats). However, measuring the response to TRH may turn out to be a extremely useful and sensitive test for this disease.

References:

Peterson ME, Broussard JD, Gamble DA: Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1994;8:279-286.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum, pp. 95-97, 2011.
Utiger RD: Tests of thyroregulatory mechanisms. In Ingbar SH and Braverman LE (eds): The Thyroid: A Fundamental and Clinical Text, pp 511-523. Philadelphia, JB Lippincott, 1986.
Tomsa K, Glaus TM, Kacl GM, et al. Thyrotropin-releasing hormone stimulation test to assess thyroid function in severely sick cats. Journal of Veterinary Internal Medicine 2001;15:89-93.
Holtman JR, Buller AL, Hamosh P, et al: Central respiratory stimulation produced by thyrotropin-releasing hormone in the cat. Peptides 1986;7:207-212.
Beleslin DB, Jovanovic-Micic D, Tomic-Beleslin N: Nature of salivation produced by thyrotropin-releasing hormone (TRH). Brain Research Bulletin 1987;18:463-465.
Beleslin DB, Jovanovic-Micic D, Samardzic R, et al: Studies of thyrotropin-releasing hormone (TRH)-induced defecation in cats. Pharmacology Biochemistry & Behavior 1987;26:639-641.
Yagi K, Ohashi E, Tanabe S, Uzuka Y, Sarashina T. Serum thyrotropin response to TRH administration in six healthy beagle dogs. Veterinary Record 2000;146:706-707.

Confirming the Diagnosis of Hyperthyroidism: The T3 Suppression Test

2011-08-30T00:23:00.000-04:00

Yawning Cat at
the Hypurrcat Spa

In most cats with hyperthyroidism, the diagnosis is easily confirmed by measuring a single serum T4 concentration. Measuring serum free T4, T3, and TSH adds more diagnostic information that can be used to help confirm the diagnosis in cats with borderline hyperthyroidism.

Occasionally, cats suspected of having hyperthyroidism can be difficult to diagnose, even when repeated serum thyroid tests are run. Many of these cats have early or mild hyperthyroidism and show only mild clinical signs, whereas others appear to have more severe clinical features of hyperthyroidism but also have another obvious (or not so obvious) concurrent disease. The finding of concomitant disease (kidney, liver, or gastrointestinal disease) is relatively common in hyperthyroid cats, which is not surprising given the fact that many of these cats are middle-aged to older.

In the majority of these hyperthyroid cats in which a normal total T4 concentration is found, simply repeating the total T4 analysis with simultaneous measurement of free T4, as well as ruling out any concurrent disease, will easily confirm the diagnosis. Further diagnostic tests for hyperthyroidism are rarely required. However, if we still still suspect that a cat has mild hyperthyroid but serum concentrations of total and free T4 are either normal or equivocal, dynamic testing can be used to help make the diagnosis.

Dynamic Testing for Diagnosis of Hyperthyroidism

Two dynamic tests can be used in cats with mild or "occult" hyperthyroidism — both of these tests act on the hypothalamic-pituitary-thyroid axis (see Figure 1) to either suppress or stimulate pituitary TSH and thyroid hormone secretion.

Triiodothyronine (T3) suppression test
Thyrotropin-releasing hormone (TRH) stimulation test

Figure 1: Hypothalamic-Pituitary-Thyroid axis
Normally, TRH stimulates the secretion of TSH from the pituitary, which in turn, stimulates the thyroid to secrete T4 and T3. Both circulating T4 and T3 "feed-back" to the hypothalamus and pituitary to shut off the secretion of TRH and TSH respectively.

Triiodothyronine (T3) Suppression Test

The T3 suppression test operates on the principle that administration of relatively high doses of exogenous T3 will suppress (lower) the secretion of pituitary TSH in normal cats. Such inhibition of pituitary TSH secretion by high circulating concentrations of thyroid hormone is a characteristic feature of normal hypothalamic-pituitary-thyroid regulation (see Figure 1). The fall in circulating TSH levels results in lowered T4 secretion from the normal feline thyroid gland (see Figure 2, below, left panel).

Because resting serum TSH concentrations are already very low in normal cats (see my previous post on serum TSH measurements in cats), it would not be possible to use TSH measurement as an endpoint for interpretation of this test.

Figure 2: T3 suppression tests in normal cats (left) & cats with hyperthyroidism (right)

In contrast to the situation in normal cats, when thyroid function is autonomous — as it is in hyperthyroid cats, administration of large doses of exogenous T3 has little or no effect on T4 secretion. The reason for this that pituitary TSH secretion has already been completely suppressed by the higher-than-normal thyroid hormone secretion characteristic of hyperthyroidism (see Figure 2 above, right panel).

Testing Protocol for the T3 Suppression Test

To perform the T3 suppression test in cats, the following protocol is recommended:

One day 1, the veterinarian draws a blood sample is drawn for determination of baseline serum concentrations of total T4 and T3. This serum sample is not yet submitted to the laboratory but kept refrigerated (or frozen) until day 4.
Owners are instructed to give 7 doses of a T3 pill (liothyronine sodium; Cytomel) to their cat, beginning on the following morning.
On day 2 and 3, the owners administer the liothyronine at a dosage of 25 µg every 8 hours for 2 days (6 doses).
On the morning of day 4, a seventh 25-µg dose of liothyronine is given and the cat returned to the veterinary clinic within 2 to 4 hours.
The veterinarian again draws a blood sample for serum T4 and T3 determinations.
Both the basal (day 1) and post-liothyronine (day 4) serum samples are submitted to the laboratory together to eliminate the effect of between assay variation in hormone concentrations.

Interpretation of Results of T3 Suppression Testing

When the T3 suppression test is performed in normal cats and sick cats without hyperthyroidism, there is a marked fall in serum T4 concentrations after exogenous T3 administration (Figures 2 and 3). In contrast, when the test is performed in cats with hyperthyroidism, even in cats with only slightly high or high-normal resting serum T4 concentrations, minimal, if any, suppression of serum T4 concentrations is seen.

Figure 3: Box plots of the serum T4 concentrations before (A) and after (B) administration of liothyronine to 44 normal cats, 77 cats with hyperthyroidism, and 22 cats with nonthyroidal disease (from data in Reference no. 4). Data is plotted as box plots, in which the "box" represents the interquartile range from the 25th to 75th percentile (represents the middle one-half of the data). The horizontal bar through the box is the median. The "whiskers" represent the main body of data, which in most cases is equal to the range. Outlying data points are represented by open circles. To convert serum T4 concentrations from nmol/L to µg/dl, divide the given values by 12.87.

Regarding interpretation of T3 suppression test results, we find that the absolute serum T4 concentration after liothyronine administration is the best means of distinguishing hyperthyroid cats from normal cats or cats with nonthyroidal disease. Cats with hyperthyroidism have post-liothyronine serum T4 values greater than 20 nmol/L (greater than 1.5 μg/dl), whereas normal cats and cats with nonthyroidal disease have T4 values less than 20 nmol/L (Figure 3B). There may be a great deal of overlap of the per cent decrease in serum T4 concentrations after liothyronine administration between the three groups of cats, but suppression of 50 per cent or more only occurs in cats without hyperthyroidism.

Serum T3 concentrations, as part of the T3 suppression test, are not useful in the diagnosis of hyperthyroidism per se. However, these basal and post-liothyronine serum T3 determinations can be used to monitor owner compliance with giving the drug. If inadequate T4 suppression is found, but serum T3 values do not increase after treatment with liothyronine, problems with owner compliance should be suspected and the test result considered questionable.

Disadvantages of the T3 Suppression Test

Overall, the T3 suppression test is very useful for diagnosis of mild hyperthyroidism in cats, but the test does come with disadvantages:

T3 suppression testing is a relatively long test (3 days)
Owners are required to give multiple doses of liothyronine
Cats must swallow the tablets if the test is going to be meaningful.

If the liothyronine is not administered properly or the cat does not swallow the liothyronine tablet, circulating T3 concentrations will not rise to decrease pituitary TSH secretion, and the serum T4 value will not be suppressed, even if the pituitary-thyroid axis is normal. Failure of a cat to ingest the liothyronine could result in a false-positive diagnosis of hyperthyroidism in a normal cat or cat with nonthyroidal disease.

References:

Graves TK, Peterson ME: Occult hyperthyroidism in cats. In Kirk RW and Bonagura JD (eds): Current Veterinary Therapy XI, pp 334-337. Philadelphia, WB Saunders Co, 1992.
Peterson ME, Melián C, Nichols CE: Measurement of serum concentrations of total and free T4 in hyperthyroid cats and cats with nonthyroidal disease. Journal of Veterinary Internal Medicine 1998;12:211.
Utiger RD: Tests of thyroregulatory mechanisms. In Ingbar SH and Braverman LE (eds): The Thyroid: A Fundamental and Clinical Text, pp 511-523. Philadelphia, JB Lippincott, 1986.
Peterson ME, Graves TK, Gamble DA: Triiodothyronine (T3) suppression test. An aid in the diagnosis of mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1990;4:233-238.
Refsal KR, Nachreiner RF, Stein BE, et al: Use of the triiodothyronine suppression test for diagnosis of hyperthyroidism in ill cats that have serum concentration of iodothyronines within normal range. J Am Vet Med Assoc 1991;199:1594-1601.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum TSH Concentrations

2011-08-22T21:55:00.000-04:00

Measuring TSH in Human Patients with Thyroid Disease

In human patients, measurement of circulating thyroid stimulating hormone (TSH; also called thyrotropin) is commonly used as a front-line test of thyroid function. This hormone is secreted by the pituitary gland and, as its name suggests, acts to stimulate the thyroid gland to secrete T4 and T3.

The pituitary gland constantly monitors the circulating levels of T4 and T3, and if it senses the slightest increase in serum thyroid hormone concentrations, it stops producing TSH. In contrast, if the pituitary senses even a slight decrease in circulating T4 and T4 concentrations, it increases the secretion of TSH in an attempt to increase thyroid hormone levels back to normal.

Consequently, the finding of a low to undetectable blood TSH value in a human patient is diagnostic for hyperthyroidism, whereas a high serum TSH concentration is diagnostic for hypothyroidism. When testing human patients, the finding of a normal TSH result excludes both hypo- or hyperthyroidism.

TSH Concentrations as a Diagnostic Test for Cats

A species-specific feline TSH assay has not yet been developed for use in cats, and human TSH assays cannot be used to measure feline TSH. However, assays for measuring canine TSH (cTSH) are widely available, and studies have investigated the use of cTSH measurements as a diagnostic test for cats with suspected thyroid dysfunction — both hyperthyroidism or hypothyroidism (1-3).

However, there are many problems with the use of the cTSH assay, both in dogs as well as in cats.

First Problem: The first issue or problem with the use of the canine TSH assay is that this test, although first developed in 1997 (now 14 years ago), is still a considered "first generation" assay. Overall, this assay is not considered very reliable for the following reasons:

It has a diagnostic test sensitivity of only 60-75% in canine hypothyroidism, a very common endocrine problem in dog (4-6). Because 25% to 40% of dogs with confirmed hypothyroidism do not have the high serum TSH concentrations (> 0.6 ng/ml) that we expect to find with thyroid failure, it's possible that some isoforms of the TSH hormone are not being detected with this assay.
In addition to this high incidence of false-negative results, cTSH levels appear to be falsely-high in 10% to 20% of dogs with normal thyroid function (4). Some of these dogs have other nonthyroidal illnesses; however, falsely high serum cTSH values have even been reported in clinically normal dogs that had completely normal total and free T4 concentrations.
The high prevalence of false-negative and false-positive test results make this assay unreliable to use as a sole diagnostic test for dogs with suspected hypothyroidism.

Second Problem: It is important to remember that the current canine TSH assays only detects approximately 35% of the circulating feline TSH. In other words, the current cTSH assay does not completely cross-react with feline TSH; therefore, the assay is not measuring the total amount of TSH present in the cat's serum.

This poor cross-reactivity of feline TSH in the canine assay explains why the upper limit of the reference range for TSH is so much lower in cats (0.15-0.3 ng/ml) than it is in dogs (0.5-0.6 ng/ml). Again, the cTSH assay is only measuring about a third of the feline TSH present in the circulation.

Third Problem: A major problem with the canine TSH assay, being a first generation assay, is that its detection limit (assay sensitivity) is not very good. In fact, it's really poor! Therefore, this assay does not perform well when we are trying to measure very low concentrations of TSH.

In cats this issue is again made even worse by the fact that only 35% of the cat's TSH will be detected by the cTSH assay. As a consequence of this poor cross-reactivity, the assay sensitivity, which is already considered poor at 0.03 ng/ml in the dog, equates to only 0.10 ng/ml of TSH in the cat (7). With such a "high" detection limit, we cannot reliably distinguish a normal TSH concentration from a low or undetectable value in cats with this cTSH assay. In other words, this assay just cannot accurately measure low enough to distinguish between normal and low feline TSH concentrations.

All human TSH assays currently used are second or even third generation assays. Like the cTSH assays, the first generation human TSH assays were also unable to distinguish low-normal from low TSH concentrations. The major advantages of the second to third generation TSH assays is their 10- to 100-fold improvement in assay sensitivity (8); this much lower detection limit greatly improves their ability to accurately distinguish between normal and even partially suppressed TSH results.

Hopefully, we will also be moving on to a second generation of TSH assays for dogs (and cats), which should improve the sensitivity (detection limit) of the assay. This will also help make this a better test for hyperthyroidism, as well as hypothyroidism in cats.

Using TSH Concentrations as a Diagnostic Test for Hyperthyroidism in Cats

Of course, the poor detection limit of the current cTSH assay represents a major issue in cats with hyperthyroidism, where low suppressed values are expected. In one of the best studies of cTSH concentrations in cats (2), all of the hyperthyroid cats tested had cTSH concentrations at or below the limit of detection of the assay (0.03 ng/ml). However, of the 40 cats without hyperthyroidism tested in that same study, 5 cats also had undetectable levels of TSH, indistinguishable from the values in the hyperthyroid cats (see Figure below).

TSH values in normal cats, hyperthyroid cats, and cats with chronic kidney disease. Notice that all hyperthyroid cats have very low TSH concentrations. However, there is much overlap between the TSH values in the hyperthyroid cats and the values in the cats without hyperthyroidism. Modified from data in reference 2.

Obviously, a better TSH assay for feline hyperthyroidism is needed— specifically, a feline-specific TSH assay that has adequate sensitivity to reliably distinguish a normal value from a low one. However until better TSH assays for cats are available, caution is advised in over interpreting values in cats since can be so difficult to distinguish normal values from the suppressed values expected in cats with hyperthyroidism. Perhaps the only use for TSH measurements using the cTSH assay would be to exclude hyperthyroidism, i.e., finding a mid- to high-normal value rather than a suppressed value (3).

Using TSH Concentrations as a Diagnostic Test for Hypothyroidism in Cats

At this time, the major use for the current cTSH assay is as a diagnostic test for hypothyroidism, where the low circulating thyroid hormone concentrations sensed by the pituitary gland leads to high serum TSH values.In accord with that, high cTSH values have been reported in cats with naturally occurring hypothyroidism, as well as cats with iatrogenic hypothyroidism, i.e., secondary to methimazole or radioiodine treatment (9).

Normal cats and cats with nonthyroidal illness generally maintain normal values for serum TSH. Therefore, the finding of a low total or free T4 in combination with a high TSH concentration greatly improves the diagnostic sensitivity for hypothyroidism in cats.

References:

Greco DS. Diagnosis of congenital and adult-onset hypothyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:40-44
Wakeling J, Moore K, Elliott J, et al. Diagnosis of hyperthyroidism in cats with mild chronic kidney disease. Journal of Small Animal Practice 2008;49:287-294.
Wakeling J. Use of thyroid stimulating hormone (TSH) in cats. Canadian Veterinary Journal 2010;51:33-34.
Peterson ME, Melian C, Nichols R. Measurement of serum total thyroxine, triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. Journal of the American Veterinary Medical Association 1997;211:1396-1402.
Scott-Moncrieff JC, Nelson RW, Bruner JM, et al. Comparison of serum concentrations of thyroid-stimulating hormone in healthy dogs, hypothyroid dogs, and euthyroid dogs with concurrent disease. Journal of the American Veterinary Medical Association 1998;212:387-391.
Dixon RM, Mooney CT. Evaluation of serum free thyroxine and thyrotropin concentrations in the diagnosis of canine hypothyroidism. Journal of Small Animal Practice 1999;40:72-78.
Ferguson DC, Caaffall Z, Hoenig M. Obesity increases free thyroxine proportionally to nonesterified fatty acid concentrations in adult neutered female cats. Journal of Endocrinology 2007;194:267-273.
Dunlap DB. Thyroid Function Tests. In: Walker HK, Hall WD, Hurst JW (eds). Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston, 1990.
Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum Free T3 Concentrations

2011-08-16T07:25:00.000-04:00

As I discussed in my recent posts on diagnostic testing for hyperthyroidism, the feline thyroid gland makes two active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T4 makes up 90% of the circulating thyroid hormones, while T3 makes up less than 10%. In cats, all circulating T4 originates from thyroid secretion but almost all T3 in the cat is produced extrathyroidally from T4 deiodination (1).

Although only 10% of the circulating thyroid hormone is T3, this thyroid hormone is 3-10 times more active than T4; T4 can be considered a prohormone whose main function is only to be converted into active T3. In order for the body’s cells and tissues to use more of this active form of thyroid hormone, T4 (which contains 4 iodine molecules) is converted to T3 (which contains 3 iodine molecules) by losing an iodine molecule. This function is performed in peripheral tissues (such as the liver and kidney) by deiodinases (enzymes that act to remove an iodine group from the thyroid hormone molecule). See my previous blog post on total T3 for more information.

Despite the fact that T3 is more potent than T4, use of total T3 concentrations are not a good diagnostic test for hyperthyroidism in cats. Over 30% of hyperthyroid cats have a normal serum T3 concentration even when they have clearly high total and free T4 values (2-4).

But what about free T3 concentrations? Would measuring free T3 be a better indicator of what's happening at this tissue level?

Physiology of T3, Total T3, and Free T3 in the Body

What's the difference between total and free T3? It's the same situation as with total T4 and free T4, which I discussed in a recent post.

When we measure a serum T3 level, we are checking the total amount of T3 hormone circulating in the blood—both the bound and unbound T3 molecules (5). More than 99% of T3 hormone in the circulation is “bound,” meaning that it is attached to thyroid-binding proteins in the bloodstream. When bound, this T3-thyroid binding protein complex is too large to enter the body's tissues (see Figure).

For circulating T3 to do its functions and regulate metabolism, the hormone must first break loose from its binding proteins (i.e., become “free” T3) in order to leave the bloodstream and enter the body’s tissues and cells. Only then can intracellular free T3 have its effect on the body’s metabolism (see Figure).

Total T3 circulates mostly bound to thyroid binding protein, with a small proportion being unbound or "free." It is only the free T3 that can pass into the tissues and cells to complete its functions to regulate metabolism.

Serum free T3 represents the tiny fraction (less than 1%) of T3 hormone that is unbound and therefore is biologically active. It is important to realize that a dynamic equilibrium exists between free and protein-bound T3 that depends on the amount of thyroid-binding proteins in the blood stream, as well as the affinity of the thyroid binding proteins for T3. In other words, a T3 molecule circulating in the bloodstream may be free (unbound) one minute, protein-bound the next, and free again shortly thereafter.

Again, it is only the free T3 portion of the total T3 measured in the blood that can pass into the cells and act on the body’s tissues to influence metabolism (see Figure).

Serum Free T3 Concentration as a Diagnostic Test in Hyperthyroidism

The use of total T3, as noted above, is not a very useful diagnostic test for hyperthyroidism in cats (2-4). Similarly, total T3 are rarely useful in the routine diagnosis of human patients with hyperthyroidism either (6-8). Rarely, an occasional human patient with hyperthyroidism will develop a syndrome called "T3 toxicosis" in which circulating total and free T4 concentrations remain normal but serum concentrations of total and free T3 are very high (6-8). Such a syndrome of T3 hyperthyroidism has never been reported and does not appear to develop in cats (2-4).

In hyperthyroidism, the increases in free T4 and free T3 concentrations are usually more marked than the increases in total hormone concentrations. In human patients, progressive increases in serum total T4 can eventually exceed the limited binding capacity of thyroid binding globulin, the major serum binding protein for T4 and T3 in people (5), This leads to disproportionate increases in the free serum concentrations of T4 (9) and T3 (10). Although similar studies have not been reported in cats, similar findings could be expected.

The questions remains — would determination of free T3 be of any added benefit over the use of T4 and free T4 concentrations for diagnosis of hyperthyroidism in cats? No one knows the answer for sure, primarily due to the fact that most diagnostic laboratories do not offer free T4 measurements. However, one large diagnostic laboratory, the Michigan State University's Diagnostic Center for Population and Animal Health (DCPAH) does offer complete feline thyroid profiles that include T4, T3, free T4, and free T3). Click this link to see their lab submission form.

Recently, I called Dr, Kent Refsal, an endocrinologist who has worked at the DCPAH for many years and a leading expert in this field of diagnostic endocrinology. I asked Dr. Refsal if he had any evidence that adding free T3 test as part of the "Feline Thyroid Profile" increased the diagnostic accuracy of their thyroid panel for cats with hyperthyroidism. The short answer is no — adding free T3 might help in other situations, such as monitoring thyroid hormone replacement, but it does NOT appear to add any diagnostic advantage of the use of total and free T4 concentrations in cats.

Bottom Line: Free T3 determinations, at least by themselves, do not appear to be a useful test for hyperthyroidism in cats.

References:

Foster DJ, Thoday KL, Beckett GJ. Thyroid hormone deiodination in the domestic cat. Journal of Molecular Endocrinology 2000;24:119-126.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Stockigt JR. Free thyroid hormone measurement. A critical appraisal. Endocrinology and metabolism clinics of North America 2001;30:265-289.
Dunlap DB. Thyroid Function Tests. In: Walker HK, Hall WD, Hurst JW (eds). Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston, 1990.
Klee GG. Clinical usage recommendations and analytic performance goals for total and free triiodothyronine measurements. Clinical Chemistry 1996;42:155-159.
Sapin R, Schlienger JL. Thyroxine (T4) and tri-iodothyronine (T3) determinations: techniques and value in the assessment of thyroid function. Annales de Biologie Clinique 2003;61:411-420.
Inada M, Sterling K. Thyroxine transport in thyrotoxicosis and hypothyroidism. The Journal of Clinical Investigation 1967;46:1442-1450.
Nauman JA, Nauman A, Werner SC. Total and free triiodothyronine in human serum. The Journal of Clinical Investigation 1967;46:1346-1355.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum Free T4 (Part 2)

2011-08-12T16:43:00.003-04:00

As I discussed in a recent blog post, determining a cat's serum free T4 concentration is a very useful test in confirming the diagnosis of hyperthyroidism, especially when run together with a total T4 concentration (1-6).

Physiology of T4, Total T4, and Free T4 in the Body

Again, what's the difference between total and free T4? When we measure a serum T4 level, we are checking the total amount of T4 hormone circulating in the blood—both the bound and unbound T4 molecules.

More than 99% of T4 hormone in the circulation is “bound,” meaning that it is attached to thyroid-binding proteins in the bloodstream. When bound, this T4-thyroid binding protein complex is too large to enter the body's tissues (Figure 1).

For circulating T4 to do its functions and regulate metabolism, the hormone must first break loose from its binding proteins (i.e., become “free” T4) in order to leave the bloodstream and enter the body’s tissues and cells. Only then can intracellular free T4 be converted to T3 to have its effect on the body’s metabolism (Figure 1).

Serum free T4 represents the tiny fraction (less than 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. It is important to realize that a dynamic equilibrium exists between free and protein-bound T4 that depends on the amount of thyroid-binding proteins in the blood stream, as well as the affinity of the thyroid binding proteins for T4. In other words, a T4 molecule circulating in the bloodstream may be free (unbound) one minute, protein-bound the next, and free again shortly thereafter.

Again, it is only the free T4 portion of the total T4 measured in the blood that can pass into the cells and act on the body’s tissues to influence metabolism (Figure 1).

Figure 1: T4 circulates mostly bound to thyroid binding proteins in the circulation. This bound T4 exists in a dynamic equilibrium with unbound or free T4, which accounts for less than 1% of the total T4 in the circulation. Only the free, nonprotein-bound moiety is able to pass into the body's cells, where it can be converted into T3, the most active thyroid hormone

Measuring Serum T4, T3, and Free T4 in Cats with Mild Hyperthyroidism

When we measure a serum free T4 concentration, we are checking only the free or unbound portion of the T4 hormone circulating in the blood. Since changes in the concentrations (or binding affinity) of the thyroid binding proteins does not affect the free T4 levels, this test is considered a more accurate test of true thyroid activity than determination of serum total T4.

Free T4 is much less likely to be influenced by nonthyroidal illness or drugs. However, the tests biggest advantage appears to be a more sensitive test for diagnosis of early or mild hyperthyroidism.

As might be expected, the free T4 test is a more sensitive diagnostic test for feline hyperthyroidism than is determination of the total T4 concentration.

In our study of 917 hyperthyroid cats (3), we found that determination of serum free T4 was diagnostic in 98.5% of the cases (for more information, see Figure 1 in my last blog post). This ability to confirm hyperthyroidism in cats with the disease was higher than the total T4 concentration, which was diagnostic in 91% of cats or T3, which was diagnostic on only 67%.

However, when we looked at the cats with mild hyperthyroidism in that study (3), the advantage of free T4 over of total T4 or T3 measurements was much more striking (Figure 2).

In that subgroup of 205 cats with mild hyperthyroidism, we found that determination of serum total T4 was diagnostic in 61% of the cases (left panel, yellow boxed data). Use of serum T3 concentrations were of almost no value in cats with mild disease, since they were diagnostic in only 20% of the cases (middle panel, blue boxed data). However, despite the poor test sensitivities of T4 and T3 measurements, determination of free T4 was still diagnostic for hyperthyroidism in 91% of these cats with mild hyperthyroidism (right panel, purple boxed data).

Figure 2: Box plots of serum total T4, T3, and free T4 concentrations in 205 cats with mild hyperthyroidism, defined as a serum T4 less than 5 μg/dl (66 nmol/L). The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile or middle half of the data). The horizontal bar in the box is the median or 50% percentile. Outlying data points are represented by open circles. The green shaded area represents the reference interval (normal range). From reference 3.

Disadvantages of Free T4 as a Diagnostic Test

Although the free T4 is a very sensitive diagnostic, the main problem with free T4 assays is that the test is less specific than the total T4 value. In other words, many cats suffering from other illnesses NOT associated with hyperthyroidism can have false-positive results with the free T4 test (for more information, see this blog post).

In contrast, cats with nonthyroidal illness will never have high total T4 values. Rather, the will have corresponding total T4 values in the low-normal or subnormal range. For this reason, the total T4 test remains the diagnostic test of choice for cats with suspected hyperthyroidism because we almost never see false-positive results with the total T4 test. Measurement of free T4 alone can never be used to make a reliable diagnosis because of the chance that the result is not accurate.

However, in cats in which hyperthyroidism is suspected based on clinical features (e.g., weight loss despite a good appetite, palpable thyroid tumor), the finding of a high free T4 concentration can be considered diagnostic. This is especially true when the total T4 values are in the upper half of the reference range (see Figure 2: left panel, yellow boxed data) and all other diseases have been excluded.

If the corresponding total T4 is in the lower half of the reference range, however, nonthyroidal disease is very likely. In our study (3), all 5 of the cats that had total T4 concentrations in the low-normal range had severe concurrent illnesses (see Figure 2: left panel, yellow boxed data).

References:

Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

Obesity Growing Among Dogs and Cats

2011-08-09T14:00:00.001-04:00

Obesity rates are rising among pets and more than half of cats and dogs are overweight or obese according to Association for Pet Obesity Prevention.

Obesity Facts and Risks

54% of Dogs and Cats in the United States are Overweight
21% of US Dogs and Cats are Overtly Obese
55.6% of US Dogs are Overweight
20% of US Dogs are Overtly Obese
54% of US Cats are Overweight
22% of US Cats are Overtly Obese

All of this data is taken from a 2010 study reported by theAssociation for Pet Obesity Prevention - click here to view the 2010 Pet Obesity Study.

Just as obesity does in humans, extra pounds can lead to adverse health conditions in animals. The most common side effects of obesity seen in dogs and cats include the following concurrent problems.

Primary Risks of Excess Weight in Pets

Diabetes mellitus
Insulin resistance and the metabolic syndrome
High blood pressure
Heart and respiratory diseases
Osteoarthitis and other joint problems
Kidney Disease
Many Forms of Cancer
Decreased life expectancy (up to 2.5 years)

Veterinarians advise owners looking help their pets trim down to replace traditional treats with healthier choices. It's also a good idea to switch to a lower-calorie diet, and add more exercise. To learn more, I'd strongly recommend that you go to the Association for Pet Obesity Prevention website.

Taking Your Cat to the Veterinarian: Lessening the Stress of Vet Visits for You and Your Cat

2011-08-02T14:59:00.000-04:00

The American Association of Feline Practitioners and the International Society of Feline Medicine have released an important handout for cat owners, entitled "Getting Your Cat to the Veterinarian: Reducing the Stress of Veterinary Visits for You and Your Cat."

This handout discusses the following topics that will help make veterinary visits easier for you and your cat:

Understanding your Cat’s Behavior
Helping Your Cat Become Comfortable with the Carrier
Getting an Unwilling Cat into the Carrier
Coming Home – Keeping the Peace in a Multi-Cat Household
What Type of Carriers are Best?

To see and print this handout, click here.

For my related posts on this topic, click here.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum Free T4 Concentrations

2011-07-23T23:49:00.003-04:00

Thyroxine (T4) is the main thyroid hormone secreted by the thyroid gland. As discussed in my recent post about T4 measurement, checking a cat’s serum T4 level is a very good screening test for hyperthyroidism (1-6). Over 90% of hyperthyroid cats will have a high serum T4 concentration, thereby confirming the diagnosis.

When we measure a serum T4 level, it is important to understand that we are checking the total amount of T4 hormone circulating in the blood—both the bound and unbound T4 molecules. More than 99% of T4 hormone is “bound,” meaning that it attaches to proteins in the blood. For circulating T4 to do its functions and regulate metabolism, the hormone must first break loose from its binding proteins (i.e., become “free” T4) in order to leave the bloodstream and enter the body’s tissues and cells. Only then can intracellular free T4 have its effect on the body’s metabolism.

Therefore, measuring a total T4 concentration by itself can sometimes be misleading, because the total T4 is affected by changes in the amount of circulating thyroid binding proteins or the binding affinity of these proteins for thyroid hormone. Such changes in the binding of T4 can occur in cats (and dogs) treated with certain drugs (e.g., glucocorticoids), as well as with many nonthyroidal illnesses (e.g., kidney disease, liver, disease, intestinal disease, or cancer).

In hyperthyroid cats, administration of drugs or other concurrent illness can lower the total T4 concentration. In some hyperthyroid cats, the high serum T4 concentration may be suppressed enough to actually fall into the normal range. In this scenario, use of a total T4 determination would fail to diagnose hyperthyroidism. In addition, some cats with mild or early hyperthyroidism will also have normal serum T4 concentrations and can be difficult to diagnose.

Physiology of Free T4 in the Body

Serum free T4 represents the tiny fraction (less than 0.1%) of thyroxine hormone that is unbound and therefore is biologically active. Again, it is only the free T4 portion of the total T4 measured in the blood that can pass into the cells and act on the body’s tissues to influence metabolism.

When we measure a serum free T4 concentration, we are checking only the free or unbound portion of the T4 hormone circulating in the blood. Since changes in the thyroid binding proteins in the blood do not affect the free T4 levels, this test is considered a more accurate test of true thyroid activity than determination of serum total T4. Free T4 is much less likely to be influenced by nonthyroidal illness or drugs. And finally, it appears to be a more sensitive test for diagnosis of early or mild hyperthyroidism.

Measuring the Serum Thyroxine (T4) Concentration: Advantages as a Diagnostic Test

As might be expected, the free T4 test is a more sensitive diagnostic test for feline hyperthyroidism than is determination of the total T4 concentration.

In our study of 917 hyperthyroid cats (3), we found that determination of serum free T4 was diagnostic in 98.5% of the cases (see Figure below; middle blue boxed data). This ability to confirm hyperthyroidism in cats with the disease was higher than the total T4 concentration, which was diagnostic in 91% of cats (see Figure 3 in my previous blog on T4 measurement).

The free T4 was high in most cats with mild or early hyperthyroidism, even when the total T4 was within the established normal range. In addition, in hyperthyroid cats with concurrent diseases or in those treated with drugs, the free T4 is not suppressed and remains high even when the total T4 has been suppressed. Therefore, use of the free T4 is a very useful diagnostic test, especially in hyperthyroid cats in which total T4 values are within reference range limits.

Figure 1: Box plots of Free T4 concentrations in 172 clinically normal cats, 917 cats with hyperthyroidism, and 221 cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 3.

Measuring the Serum Thyroxine (T4) Concentration: Disadvantages as a Diagnostic Test

So if determination of the free T4 level is more diagnostic, why bother with running the total T4 at all? Why not measure just measure free T4 concentrations as the main diagnostic test for hyperthyroidism?

It turns out that there are 2 good reasons why free T4 concentrations cannot replace total T4 as the main diagnostic test for hyperthyroid cats.

First of all, although the free T4 is a very sensitive diagnostic, the main problem with free T4 assays is that the test is less specific than the total T4 value. In other words, many cats suffering from other illnesses NOT associated with hyperthyroidism can have false-positive results with the free T4 test (see Figure above; right purple boxed data).

Of the 221 cats with nonthyroidal disease in my study, 14 cats (6.3%) had a falsely high free T4 concentration (see Figure above; right purple boxed data; Notice all of the outlying data points above the normal range). Therefore, calculated specificity of measuring serum free T4 concentration as a diagnostic test for hyperthyroidism was significantly lower than that of the total T4 concentration.

Remember that these cats with nonthyroidal illness will NEVER have high total T4 values — rather, as expected, the have corresponding total T4 values in the low-normal or subnormal range (see Figure 3 in my previous blog on T4 measurement).

For this reason, the total T4 test remains the diagnostic test of choice for cats with suspected hyperthyroidism because we almost never see false-positive results with the total T4 test. Measurement of free T4 alone can never be used to make a reliable diagnosis because of the chance that the result is not accurate.

The second reason why free T4 concentrations should not be used as the initial screening test for hyperthyroidism is that the free T4 test is generally about 2 to 3 times as expensive as a T4 alone.

While most endocrinologists favor the standard equilibrium dialysis radioimmunoassay method for measuring free T4, newer analog and chemiluminescence assays now offer alternative and accurate methodologies. These new assays are also faster to run and are less costly.

No matter how free T4 is determined, however, the free T4 test must be run with a total T4, adding to the expense of diagnostic testing for hyperthyroid cats.

Bottom Line: Caution is advised in using serum free T4 measurements as the sole diagnostic test for hyperthyroidism. This test is associated with a high rate of false-positive results in sick cats without hyperthyroidism. This test should never be run by itself but it is more reliable if interpreted with a corresponding total T4 value:

High-normal total and free T4 concentrations are generally consistent with hyperthyroidism (especially is clinical features of disease present— i.e., thyroid nodule, weight loss despite good appetite)
Low to low-normal total T4 concentrations together with a high free T4 are usually associated with non-thyroidal illness

References:

Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.
Peterson ME, Broome MR, Robertson J: Accuracy of serum free thyroxine concentrations determined by a new veterinary chemiluminescent immunoassay in euthyroid and hyperthyroid cats. Proceedings of 2011 European College of Veterinary Internal Medicine (in press)

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum T3 Concentrations

2011-07-17T15:11:00.003-04:00

As I discussed in my last post, the feline thyroid gland makes two active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T4 makes up nearly 90% of the circulating thyroid hormones, while T3 makes up less than 10%.

Although only 10% of the hormone produced in the thyroid gland is T3, this thyroid hormone is three to ten times more active than T4. In order for the body’s cells and tissues to use more of this active form of thyroid hormone, T4 (which contains four iodine molecules) is converted to T3 or (which contains three iodine molecules) by losing an iodine molecule. In cats, this function is performed in the liver and kidney by deiodinase enzymes that act to remove an iodine group from the thyroid hormone molecule (1).

The deiodination of T4 to T3, reverse T3, and other iodothyronines is an integral component of thyroid hormone metabolism. This deiodination, depending on whether it occurs at the outer or inner rings of the T4 molecule, serves to either activate or inactivate this hormone (see Figure above). Again, T3 is the most active thyroid hormone, whereas reverse T3 (rT3) is considered to be metabolically inactive.

There are 3 different thyroid deiodioinase enzymes:

Iodothyronine deiodinase type I (IDI) is commonly found in the liver and kidney. It is also present in the thyroid of many species including dogs and humans, but IDI is not present in the feline thyroid gland (1).
The type II enzyme (IDII) in skeletal muscle and thyroid gland (but again, cats do not appear to have IDII in their thyroid gland).
Type III (IDIII) is found in the fetal tissue and brain matter.

IDII can only deiodinate the outer ring of T4 or rT3. IDIII can only deiodinate the inner ring of T4 or T3. IDI can deiodinate both the outer and inner rings.

But the real question is this: If T3 is three to ten times more active than T4, shouldn’t we be measuring serum T3 to better diagnose hyperthyroidism in cats?

Measuring the Serum Triiodothyronine (T3) Concentration

Although T3 is the important thyroid hormone for regulation of a cat's metabolic state, this hormone reflects tissue thyroid activity. Unfortunately, measuring the serum levels of T3 are a poor reflection of what is happening in the tissues.

Compared to the serum T4, measuring the serum T3 concentration is much less useful as a diagnostic test for hyperthyroidism (2-7). Over 30% of hyperthyroid cats have normal circulating T3 concentrations (See Figure, below).

Box plots of total T3 concentrations in 172 clinically normal cats, 917 cats with hyperthyroidism, and 221 cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 3.

None of the cats with high serum T3 values have normal serum T4 concentrations, so measuring T3 test does not provide any additional information not gained from measuring T4 alone.

Bottom line: Measuring total T3 alone is not recommended for investigation of hyperthyroidism in cats.

The reason for this is unknown, but based on studies of normal cats (1), it is likely that most of the circulating T3 in these hyperthyroid cats comes from conversion from T4 in peripheral tissues, and is not secreted directly by the thyroid tumor. That could explain why measurement of T4 is a better diagnostic test for hyperthyroidism than is T3.

In my next posts, we will discuss the use of free T4, then look at the use of serum TSH measurements. I’d end this section on diagnostic testing with a post on the value of thyroid scintigraphy (thyroid nuclear imaging or scanning) in the diagnosis of hyperthyroidism.

References:

Foster DJ, Thoday KL, Beckett GJ. Thyroid hormone deiodination in the domestic cat. Journal of Molecular Endocrinology 2000;24:119-126.
Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

Confirming the Diagnosis of Hyperthyroidism in Cats: Serum T4 Concentrations

2011-07-14T20:23:00.003-04:00

Figure 1: Palpating a cat's neck
for a thyroid tumor

As I've discussed in my last few blood posts, we base our preliminary or initial diagnosis of hyperthyroidism on a combination of the three factors or findings:

Your cat's clinical features (e.g., weight loss despite a good appetite).
Your cat's physical examination findings (e.g., rapid heart rate, enlarged thyroid tumor).
The findings on routine laboratory testing (i.e., complete blood count, chemistry panel, and urinalysis).

If, after reviewing the cat's history and exam finding, we still believe that your cat has hyperthyroidism, the next step in the workup is to do specific tests to confirm the diagnosis of hyperthyroidism. This allows the veterinarian to properly treat your cat for the correct problem.

We have three different groups of tests that can be used to confirm as diagnosis of hyperthyroidism:

Basal or single blood tests, in which we measure one or more of the thyroid hormones themselves (e.g., T4 or T3), or the pituitary hormone called thyroid-stimulating hormone (TSH). As its name implied, TSH plays an important role in stimulating the thyroid gland to secrete the thyroid hormones.
Dynamic testing, in which 2 or more blood samples are collected before and hours-to-days after administration of a thyroid-stimulating or suppressive agent. These tests check the integrity of the pituitary-thyroid axis, which is malfunctioning in cats with hyperthyroidism.
Imaging techniques such as thyroid scintigraphy provide a visual display of the structure and function of the thyroid gland. This is considered by most to be the “gold standard” diagnostic test since thyroid scanning will allow the diagnosis of hyperthyroidism before laboratory tests are abnormal. Thyroid scanning can also exclude a misdiagnosis of hyperthyroidism in cats with false thyroid hormone elevations.

In this first blog post in this series of “Confirming the Diagnosis of Hyperthyroidism in Cats,” I'm going to discuss the use of serum thyroxine (T4) concentrations to make a definitive diagnosis of hyperthyroidism in cats.

In my next posts, we will discuss the use of free T4, then look at the use of serum TSH measurements. I will then turn to when we do dynamic thyroid function testing. Finally, I’d end this section on diagnostic testing with a post on the value of thyroid scintigraphy (thyroid nuclear imaging or scanning) in the diagnosis of hyperthyroidism.

How the Thyroid Works —Production of Its Hormones

Figure 2: T4 with its
four iodine groups

A unique role of the thyroid gland is to take iodine supplied in food and use it to produce the 2 major thyroid hormones, thyroxine (tetraiodothyronine or T4) and triiodothyronine (T3). Important components of both the T4 and T3 molecules include the amino acid tyrosine and iodine (see Figure 2).

Inside the thyroid gland, T4 and T4 are bound to a large storage protein called thyroglobulin. When the body requires thyroid hormone, the thyroid gland releases stored T4 and T3 into circulation. T4 makes up nearly 90% of the circulating thyroid hormones, while T3 makes up less than 10%.

Because thyroid hormone synthesis requires iodine, either a deficiency or an excess of iodine can profoundly affect thyroid function, and promote autoimmune thyroiditis.

Measuring the Serum Thyroxine (T4) Concentration

The test to measures the total amount of T4 in your cat’s blood should always be the first diagnostic test used if hyperthyroidism is suspected.

The finding of a high serum T4 concentration is the hallmark of hyperthyroidism. As a diagnostic test for hyperthyroidism, the total T4 is a highly specific test. Falsely high T4 values (false-positive test results) generally are not seen unless overt laboratory errors in measurement are responsible, and that is extremely rare. Methods for T4 measurement are readily accessible, relatively cheap, and only require a small blood sample.

Figure 3: Box plots of total T4 concentrations in normal cats, hyperthyroid cats, and cats with nonthyroidal disease. The T-bars represent the main body of data. The box represents the interquartile range (25th percentile to 75th percentile range or the middle half of the data). The horizontal bar in the box is the median. Outlying data points are represented by open circles. The shaded area indicates the reference interval (normal range). From reference 9.

Serum total T4 is a great screening test for hyperthyroidism. The total T4 assay has a test sensitivity of over 90%; in other words, 90% of all hyperthyroid cats can be diagnosed by measuring a T4 concentration (Figure 3, above).

However, this means that approximately 10% of cats with hyperthyroid will have serum total T4 concentration that are not high but remain within the normal, reference range limits. When “normal” T4 values are found, they are usually within the high-normal range.

Thus, a high total T4 value is diagnostic for hyperthyroidism in cats, especially if you suspect hyperthyroidism based upon the cat’s clinical features and physical exam findings. However, if hyperthyroidism is suspected but the serum T4 test is normal, it is still possible you’re your cat has the disease.

What how can the serum T4 be normal if a cat is hyperthyroid?

There are 2 main reasons or situations where this can occur:

In early or mildly affected hyperthyroid cats, serum total T4 concentrations can fluctuate in and out of the reference range. Such fluctuation occurs in all hyperthyroid cats, but the degree of fluctuation is of little diagnostic significance in cats with moderate to advanced hyperthyroidism and markedly high T4 concentrations.
Other illness or disease (not related to hyperthyroidism) is capable of suppressing serum total T4 from high concentrations to within the mid- to high-end of the reference range. Concurrent hyperthyroidism should always be suspected in severely ill cats with mid to high reference range serum total T4 concentrations.

What do we do if the serum T4 is normal and hyperthyroidism is still suspected?

In most cats with early or mildly hyperthyroidism, serum total T4 concentrations will eventually increase into the diagnostic hyperthyroid range with time. So one approach to cats like this is to simply wait and retest a few weeks later.

Similarly, after a hyperthyroid cat recovers from the nonthyroidal illness, the serum T4 values will again increase in the high range, confirming the diagnosis. So concurrent disease should always be excluded in any cat suspected of having hyperthyroidism when the serum T4 concentration is normal.

With time, almost all hyperthyroid cats will eventually develop high T4 concentrations.

Other diagnostic tests may also be used provide a means of diagnosing hyperthyroidism in cats that have a normal T4 test. These options include measurement of free T4, TSH, dynamic thyroid function testing, or thyroid scintigraphy (all discussed in my next upcoming blog posts).

References:

Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press.
Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:10-16.
Graves TK, Peterson ME. Diagnostic tests for feline hyperthyroidism. The Veterinary Clinics of North America: Small Animal Practice 1994;24:567-576.
Kintzer PP, Peterson ME. Nuclear medicine of the thyroid gland. Scintigraphy and radioiodine therapy. The Veterinary Clinics of North America: Small Animal Practice 1994;24:587-605.
Peterson ME, Graves TK, Cavanagh I. Serum thyroid hormone concentrations fluctuate in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1987;1:142-146.
Peterson ME, Graves TK, Gamble DA. Triiodothyronine (T3) suppression test. An aid in the diagnosis of mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1990;4:233-238.
Peterson ME, Gamble DA. Effect of nonthyroidal illness on serum thyroxine concentrations in cats: 494 cases (1988). Journal of the American Veterinary Medical Association 1990;197:1203-1208.
Peterson ME, Broussard JD, Gamble DA. Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyroidism in cats. Journal of Veterinary Internal Medicine 1994;8:279-286.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. Journal of the American Veterinary Medical Association 2001;218:529-536.
Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. pp. 95-97, 2011
Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, in press.

The Animal Endocrine Clinic Announces Its New Monthly Newsletter

2011-07-04T14:57:00.000-04:00

We've just emailed the first of our monthly Animal Endocrine Clinic (AEC) newsletters. A snapshot of the July 2011 AEC Newsletter appears above, and the full Newsletter can be read online here.

If you'd like to sign up for the AEC Newsletters, please click through to the sign-up page.

For more information about AEC visit www.animalendocrine.com.

Become a fan of AEC on Facebook (www.facebook.com/animalendocrine)
Follow AEC’s veterinary blog (http://endocrinevet.blogspot.com/)
Follow AEC on Twitter (http://twitter.com/#!/animalendocrine)
Follow AEC on YouTube: (http://www.youtube.com/animalendocrine)

Check Out the Animal Endocrine Clinic's New Website

2011-07-04T13:51:00.000-04:00

PRESS RELEASE: JUNE 30, 2011

The Animal Endocrine Clinic Announces the Launch of its Redesigned Website

Dr. Mark Peterson and the Animal Endocrine Clinic launch the most comprehensive, up-to-date website on the study of endocrinology in dogs and cats

The Animal Endocrine Clinic (AEC) has announced the launch of its totally redesigned website. The website was developed as a resource for pet owners and veterinarians to learn more about Dr. Peterson, his specialty veterinary clinic, and about endocrine disorders that commonly affect dogs and cats.

Featuring a new logo, the website combines AEC’s three divisions: Hypurrcat, the Endocrine Clinic, and Nuclear Imaging for Animals. The website also reflects the sentiments of dog and cat owners and veterinarians seeking more information about the diagnosis and treatment of endocrinology disorders.

Dr. Mark E. Peterson, owner and founder of the Animal Endocrine Clinic says, “As our practice has grown, and we have received calls and emails from both pet owners and veterinarians, we realized the need for an online resource for those who want to understand more about these common pet afflictions. Our new website does just that.”

Highlights of the Redesign Include:

Endocrine Resources — A comprehensive look at the study of endocrinology in dogs and cats including an overview of both animals’ endocrine systems, specific endocrine diseases that affect each animal, symptoms to look for, and the most comprehensive online glossary of terms available.

Also included are descriptions of the practice’s divisions and the cutting edge technology that AEC uses to diagnose and treat these disorders.

Publications — A world-renowned specialist in feline and canine endocrinology, Dr. Mark E. Peterson was the first veterinarian to document hyperthyroidism in cats as well as the first to treat hyperthyroid cats with radioiodine. He has also published extensively about diabetes mellitus and adrenal disease in dogs and cats. A list of his published writings have been archived on the website.

Events — Dr. Peterson continues to be very involved in organized veterinary medicine and veterinary continuing education events. A schedule of his current and past speaking engagements is maintained on the new site.

Integrated Social Media — The new website includes videos, testimonials and allows users to access all of the AEC social media directly from the AEC website.

“The new design provides a cleaner, user experience while integrating our work at AEC with the expanding social media market,” said Dr. Peterson. “We now have the opportunity to deepen existing relationships by further engaging our audience, while also providing access to timely and quality information which has become more important to pet owners and veterinary colleagues.”

About The Animal Endocrine Clinic

The Animal Endocrine Clinic (AEC) is a specialized veterinary practice that diagnoses and treats cats and dogs with endocrine (hormonal) disorders.

AEC has three divisions: Hypurrcat exclusively treats hyperthyroid cats with radioactive iodine; the Endocrine Clinic is dedicated to diagnosing and treating dogs and cats with endocrine disorders, such as diabetes or Cushing's disease; and Nuclear Imaging for Animals is a state-of-the-art medical imaging facility that performs nuclear scanning to diagnose thyroid, bone, liver, and kidney diseases in dogs and cats.

AEC has clinics in Manhattan and Westchester County and treats clients in New York City, Long Island, Westchester County, New Jersey and Connecticut. It is the only practice of its kind in the United States.

For more information about AEC visit www.animalendocrine.com.

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Diagnosing Hyperthyroidism in Cats: Routine Testing Procedures

2011-06-22T12:24:00.000-04:00

A variety of standard testing procedures are recommended in the workup of cats suspected of suffering from hyperthyroidism. These tests in include a complete blood count, serum chemistry analysis, and urinalysis.

Often this database simply lends support to the diagnosis, but these screening tests should always be done. They are essential in determining if a hyperthyroid cat also has concurrent medical problems (e.g., kidney, heart, or liver disease). These conditions may affect which form of treatment is recommended, as well as providing a more accurate prognosis.

Many hyperthyroid cats will develop secondary heart issues, including a rapid heart rate, heart murmurs, heart enlargement, or abnormal heart rhythms. Therefore heart procedures, such as a chest radiograph (x-ray), electrocardiogram (ECG), or echocardiogram, may also be important in the workup of a cat with suspected hyperthyroidism.

Routine Bloods Testing

Complete blood count
Hematological findings are usually non-specific and mostly not clinically important.

An increased number of red blood cells (erythrocytosis with increased packed cell volume (PCV), red blood cell (RBC) count and hemoglobin concentration) are relatively common. These changes reflect thyroid hormone stimulation of the bone marrow.
A high white blood cell (WBC) count associated with a stress leukogram is common. With this pattern, which is associated with clinical states of stress, we see high numbers of neutrophils accompanied by low numbers of lymphocytes and eosinophils.
Anemia, when present, is almost never caused by the hyperthyroid state and a search for another cause should be undertaken.

Serum chemistry panel
Serum biochemistry abnormalities are common in cats with hyperthyroidism (see Table).

Mild to marked increases in the serum activities of many liver enzymes, including alanine aminotransferase (ALT) and alkaline phosphatase (ALP) are the most common and striking biochemical abnormalities of feline hyperthyroidism. These liver enzymes changes and T4 concentrations are related, with liver enzyme abnormalities being more common in cats with severe hyperthyroidism. These high liver enzymes return to normal upon successful treatment of hyperthyroidism.
Before treatment, mild to moderate increases in serum concentrations of the kidney tests, urea nitrogen and creatinine, may be found in just over 20% of hyperthyroid cats. Such a prevalence of concurrent renal dysfunction or chronic kidney disease (CKD) is not unexpected in a group of older or aged cats.
These abnormalities, particularly the high urea concentration, may be exacerbated by the increased protein intake and protein catabolism of hyperthyroidism. On the other hand, in hyperthyroid cats without concurrent CKD or azotemia, circulating creatinine concentrations are lowered, which may be related in part to a loss of muscle mass.
However, this lowering of serum creatinine (and urea in some cats) is primarily the result of the increase in glomerular filtration rate (GFR) that occurs in hyperthyroid cats. These effects have implications in assessing the presence of primary renal dysfunction in hyperthyroid cats (common to see “masked” kidney disease in untreated hyperthyroid cats that only becomes apparent after treatment).

A number of other changes can develop in hyperthyroid cats but are either clinically insignificant or rarely encountered.

A high serum phosphate concentration without evidence of kidney disease occurs in approximately 35-45% of cats.
Blood glucose (sugar) concentrations may be mildly increased presumably reflecting a stress response. In cases with pre-existing diabetes mellitus, accelerated insulin catabolism increases requirements for exogenous insulin.
A low serum potassium (hypokalemia) has occasionally been associated with hyperthyroidism and should be suspected in any cat with evidence of severe muscle weakness.

Complete urinalysis
Urinalysis is generally unremarkable but is useful in differentiating other diseases with similar clinical signs such as diabetes mellitus.

The urine specific gravity is variable, but concurrent (masked) renal disease should be considered in all cats that have dilute urine (urine specific gravity, less than1.040). Cats with concurrent CKD can occasionally have more concentrated urine values (specific gravity, greater than 1.040), but most cats have less concentrated urine.
Mild increases in urine protein excretion (proteinuria) is common. Proteinuria found on routine urinalysis should be confirmed by measuring the urine protein-to-creatinine ratio (UPC ratio).
Normal cats have a UPC ratio of less than 0.2, where many hyperthyroid cats have borderline high (0.2-0.4) or overt proteinuria (greater than 0.4). Such proteinuria resolves upon successful treatment of hyperthyroidism.
Urinary tract infections occur in approximately 10% of hyperthyroid cats. As a consequence, urine culture is indicated in the workup of cats with hyperthyroid cats.

Table 1: Routine test abnormalities associated with hyperthyroidism in cats

Complete blood count

High red cell count
High white blood cell count
Low numbers of lymphcytes
Low numbers of eosinophils
Low red cell count (anemia) — very rare

Serum chemistry profile

High liver tests (ALT and ALP)
High kidney tests
High blood sugar (glucose)
High phosphorus

Complete urinalysis

Dilute or concentrated urine
High urine protein excretion
Infection (white blood cells, bacteria)

Heart Testing Procedures

Chest Radiographs (x-rays)

On thoracic radiography, mild to severe heart enlargement (cardiomegaly) is evident in about half of hyperthyroid cats. In the vast majority of cats, this cardiomegaly is reversible with correction of the hyperthyroid state.
With severe hyperthyroidism, cats may develop congestive heart failure. In these cats, chest x-rays may show evidence of a buildup of fluid around the lungs (pleural effusion) or within the lungs themselves (pulmonary edema).

Echocardiogram

The most common echocardiographic findings in hyperthyroid cats include left ventricular hypertrophy (thickening), left atrial and ventricular dilation, and interventricular septum hypertrophy.
However, most of these changes are subtle and are of little clinical relevance.
Increased cardiac contractility is common and invariably normalizes upon successful treatment of the hyperthyroidism.

Echocardiogram

A rapid heart rate (sinus tachycardia) develops in over half of hyperthyroid cats.
Other changes seen on the electrocardiogram (ECG) may include increased R-wave amplitude, prolonged QRS duration, shortened Q-T interval, and intraventricular conduction disturbances.
In severe hyperthyroidism, a variety of atrial and ventricular arrhythmias can develop.

Confirming the Diagnosis

Confirming the diagnosis of hyperthyroidism is usually straightforward and is done by performing specific thyroid function tests. I’ll cover this topic in my next post.

However, screening serum biochemistry and hematology remain extremely important to assess concurrent conditions that may impact the management if hyperthyroidism is diagnosed, as well as provide baseline values for parameters that can be affected by treatment (such as the kidney function tests).

References:

Bond BR, Fox PR, Peterson ME, et al. Echocardiographic findings in 103 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1988; 192:1546-1549
Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995; 206:302-305.
DiBartola SP, Brown SA. The kidney and hyperthyroidism. In: Kirk’s Current Veterinary Therapy XIII, ed. JD Bonagura, WB Saunders, Philadelphia, 2002; pp. 337-339.
Fox PR, Peterson ME, Broussard JD. Electrocardiographic and radiographic changes in cats with hyperthyroidism: comparison of populations evaluated during 1992--1993 vs 1979-1982. Journal of the American Animal Hospital Association 1999;35: 27-31.
Horney BS, Farmer AJ, Honor DJ, et al. Agarose gel electrophoresis of alkaline phosphatase isoenzymes in the serum of hyperthyroid cats. Veterinary Clinical Pathology 1994;23: 98-102.
Jacobs G, Hutson C, Dougherty J, et al. Congestive heart failure associated with hyperthyroidism in cats. Journal of the American Veterinary Medical Association 1986;188:52-56.
Nemzek JA, Kruger JM, Walshaw R, et al. Acute onset of hypokalaemia and muscular weakness in four hyperthyroid cats. Journal of the American Veterinary Medical Association 1994;205:65-68.
Peterson ME, Keene B, Ferguson DC, et al. Electrocardiographic findings in 45 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1982;180:934-937.
Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183: 103-110.
Reusch CE, Tomsa K. Serum fructosamine concentration in cats with overt hyperthyroidism. Journal of the American Veterinary Medical Association 1999;215:1297-1300.
Thoday KL, Mooney CT. Historical, clinical and laboratory features of 126 hyperthyroid cats. Veterinary Record 1992;131:257-264.

Top 12 Physical Exam Findings in Cats with Hyperthyroidism

2011-06-07T22:59:00.117-04:00

Hyperthyroidism is the clinical syndrome caused by overproduction of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). In cats, hyperthyroidism is most commonly due to a benign tumor affecting one or both lobes of thyroid gland.

Thyroid hormones normally regulate the body’s metabolic processes. As circulating levels of thyroid hormones rise higher and higher in cats with hyperthyroidism, a progressive increase in their metabolic rate develops: this leads to a multitude of changes, including weight loss and muscle wasting. High thyroid hormone concentrations also interact with and stimulate the central nervous system, which commonly leads to increased activity, restlessness, aggression, and panic attack.

Clinical features associated with hyperthyroidism can be quite dramatic and cats can become seriously ill with this condition. Untreated, hyperthyroidism in cats can lead to heart or kidney failure and can be fatal.

When you take your cat to your veterinarian, there are a number of findings that should alert us to that fact that your cat might be hyperthyroid and that specific diagnostic testing should be undertaken.

If Your Cat Is Hyperthyroid, What Will Your Veterinarian Find on the Physical Examination?

Here I’ve listed the top 12 most common physical exam findings that a veterinarian may find on routine physical examination of the feline hyperthyroid patient. Your cat doesn’t need to have all of these physical exam abnormalities in order to have an overactive thyroid problem. But if your cat has one or more of these findings, especially if he or she is older than 10 years of age, additional testing to rule out hyperthyroidism may be warranted.

1. Enlarged thyroid gland (Goiter)

Because of its small size and flat shape (only 2-3 mm thick), even an experienced veterinarian cannot palpate (feel) the normal feline thyroid gland. In contrast, palpable nodular enlargement of the thyroid gland (goiter) is present in the vast majority of cats with hyperthyroidism.

The thyroid gland is made up of 2 separate halves or lobes. About 70% of hyperthyroid cats have enlargement of both thyroid lobes (bilateral tumors), whereas 30% have involvement of only one side (unilateral tumor).

A palpable goiter is an expected finding because since all hyperthyroid cats have a tumor as the underlying cause of the disease. Almost all of these tumors are benign (i.e., adenomas) but malignant tumors (i.e., carcinomas) develop in about 5% of the cats with hyperthyroidism.

Veterinarians use a variety of techniques to palpate an enlarged thyroid gland in cats. With the classic palpation technique, the cat is placed in sitting position and the front legs held still. The neck of the cat is extended, and the veterinarian’s thumb and index finger are placed on each side of the trachea and swept downwards from the larynx to the sternum. Palpation of a freely movable, subcutaneous nodule or a “blip” that slips under the fingertips determines the presence of a goiter.

Thyroid lobes are loosely attached to the surrounding tissues and tend to migrate ventrally (downward toward the chest) as they grow and become larger. In fact, these large thyroid tumors occasionally are pulled down from their normal location adjacent to the larynx (voice box) into the chest cavity.

In some cats with very large thyroid tumors, the goiter is visible and palpation is not even required to identify it.

2. Muscle wasting
Weight loss despite a normal to increased appetite is the classic and most common sign of hyperthyroidism in cats. These cats lose weight because their metabolic rate is increased: they are burning up their food calories faster than they can consume their daily meal.

The weight loss characteristic of hyperthyroidism is generally progressive and can usually be first noticed as a loss of muscle mass around the cat’s back (spine). Despite this muscle atrophy, most hyperthyroid cats retain their “belly” during the initial stages of their thyroid disease.

With time, however, severe muscle wasting, emaciation, cachexia, and death from starvation can occur if the cat’s hyperthyroidism is left untreated. Concomitant with the progressive muscle wasting, cats with advanced or chronic hyperthyroidism may become weak and are easily fatigued.

3. Hyperactive, Nervous behavior
Hyperactivity or restlessness is relatively common in cats with hyperthyroidism. These signs may be more obvious when attempts are made to restrain the cat and are therefore often more noticeable to the veterinarians during the physical exam than to owners themselves. Many of these cats do not wish to stay on the exam table or will tolerate being handled for only short periods.

See video of severe anxiety in a cat with severe untreated hyperthyroidism here:

4. Too thin (Low body condition score)
Weight loss despite a normal to increased appetite is the classic and most common sign of hyperthyroidism in cats. Hyperthyroidism is so common that it should always be considered as a possibility in any middle-aged to older cats that has lost weight, even in none of the other clinical features of the disease are present.

Your veterinarian should weight your cat at the time of every examination. With this information, it is easy to document the progressive weight loss that occurs in cats with untreated hyperthyroidism.

Veterinarian can also determine your cat’s body condition score as a semi-quantitative method of evaluating body fat and lean body tissue. To score your cat’s body condition, the veterinarian checks for palpable fat covering the ribs, the degree of abdominal tuck, and the presence and degree of the abdominal fat pad. Because they lose too much weight, cats with advanced hyperthyroidism obviously will develop a low body condition score.

5. Fast heart rate
It is very common for hyperthyroid cats to have an abnormally fast heart rate, especially when examined at the veterinarian’s office. At home, the normal cat's relaxed heart rate is 140 to 180 beats per minute (bpm). Even in a normal cat, however, the heart rate will often be slightly fast in the clinic or hospital situation due to stress or fear.

About half of cats with hyperthyroidism will have very rapid heart rates (over 250 bpm) found at time of their physical examination. Some of these cats will even have heat rates over 200 bpm when they are relaxed at home.

In addition to the rapid heart rate, the intensity of each heartbeat is often more pronounced in cats with hyperthyroidism. In some of these cats, each heartbeat can easily be felt by placing ones hand on the cat’s chest cavity. When the veterinarian listens to a hyperthyroid cat’s heart, one hears the intense “pounding” of the overactive heart.

6. Low-grade fever
The high metabolic rate of hyperthyroid cats sometimes causes hyperthyroid cats to have a mildly elevated rectal temperature (103 F, 39.4 C). Such low grade “fever” can easily occur during a visit to your veterinarian simply due to the stress of the visit.

Almost none of these hyperthyroid cats have a true fever. Rather, the slightly high body temperature reflects the heat intolerance resulting from the increased metabolic rate of hyperthyroidism.

7. Heart murmurs
Cardiac murmurs are detected in about half of hyperthyroid cats on the physical examination. Hyperthyroidism is the single most important factor for the development of murmurs in older cats.

8. Panting or difficulty breathing

Respiratory abnormalities are common findings on physical examination and may include a rapid respiratory rate, panting, or difficulty in breathing at rest. Because of their increased metabolic rate, hyperthyroid cats generate more body heat than normal. Because of that, they are more sensitive to higher temperatures than they were when not hyperthyroid.

Cats do sweat, but not how you might think. They only sweat through their paws, but this is too small a surface area to do much cooling. Because of their decreased ability to sweat when very hot, they must breath faster (or pant) in order to dissipate excess body heat.

These respiratory signs tend to occur most frequently during periods of stress, such as a car ride or visit to the veterinarian’s office.

When severely stressed, cats with hyperthyroidism may develop severe respiratory distress, rapid breathing, and extreme panting and open-mouth breathing. During their exam or blood sampling, these cats may become extreme weak or even collapse from exhaustion and shortness of breath. Hyperthyroid cats should therefore be handled carefully and gently, especially in the veterinary office during these stressful periods.

9. Poor hair coat
Skin and hair coat changes are often detected on examination of hyperthyroid cats. The hair coat, especially in long-hair breeds, is often unkempt, dull, and may even be matted.

Long and thickened nails may also be noted, especially in cats with chronic and advanced hyperthyroidism.

10. Aggression, Panic attack
Some hyperthyroid cats will suddenly show signs of aggression during their exam. They may attempt to bite or scratch the veterinarian, technician, or even the owner during these episodes.

Even more frightening, other hyperthyroid will develop a “panic attack” as a result of the impaired tolerance for examination or restraint during blood sampling. These cats may develop panting, overt respiratory distress, weakness, and even collapse during these episodes.

11. Irregular heartbeat
An irregular heartbeat or abnormal heart rhythm (also called an arrhythmia) can occasionally be heard during the cardiac examination of cats with hyperthyroidism. Gallop rhythms can also occur.

An electrocardiogram (EGG) is commonly performed in these cats to better define the arrhythmia and determine if specific treatment is needed.

12. High blood pressure
Mild to moderate hypertension is not an uncommon finding in untreated hyperthyroid cats. However, these cats are typically only mildly hypertensive and we do not generally see severe complications associated with high blood pressure (e.g., blindness, retinal hemorrhage or detachment).

When hypertension is mild, it may simply reflect the reduced tolerance of hyperthyroid cats to stressful situations such as a veterinary examination and may explain why typical hypertensive complications are rarely seen. This is commonly referred to as “white-coat” hypertension.

Dr. Peterson Lectures at the VetCo International Congress in Poland

2011-05-30T10:53:00.001-04:00

Dr Peterson was invited to lecture at the 2011 Vetco International Congress in Warsaw, Poland, on May 21st and 22nd along with Dr. Richard W. Nelson. The theme of this year's VetCo Congress was "Endocrinology of Small Animals in Practice." Dr. Peterson delivered four lectures during the Congress:

Diagnosis and treatment of hyperthyroidism in cats
Diagnostic testing for canine hyperadrenocorticism
Treatment of hyperadrenocorticism in dogs
Addison’s disease in dogs: an overview

Dr. Peterson and Dr. Nelson both graduated from the University of Minnesota College of Veterinary Medicine in the late 1970s, when they first met and became friends. Like Dr. Peterson, Dr. Nelson has authored numerous manuscripts, book chapters and textbooks and has lectured nationally and internationally.

Dr. Peterson (left) and Dr. Nelson (right) at the 2011 VetCo Congress in Poland.

World Thyroid Day

2011-05-25T12:36:00.002-04:00

Today, May 25th 2011, we recognize the 4th Annual World Thyroid Day. Established in 2008, World Thyroid Day highlights five major goals to:

Increase awareness of thyroid health,
Promote understanding of advances made in treating thyroid diseases,
Emphasize the prevalence of thyroid diseases,
Focus on the urgent need for education and prevention programs, and
Expand awareness of new treatment modalities.

In humans, dogs and cats, the thyroid, a large endocrine gland located in the neck, produces hormones that influence virtually every cell, tissue and organ in the body. The thyroid gland regulates the body's metabolism — the rate at which the body produces energy from nutrients and oxygen — and affects critical body functions, such as energy level and heart rate.

Diseases of the thyroid gland include hyperthyroidism, hypothyroidism, and thyroid cancer. These disorders are very common and affect tens of millions of people worldwide. As we all know from reading this blog, the same thyroid diseases are also common in dogs and cats.

For more information, see: www.worldthyroidday.com.

Top 10 Signs of Hyperthyroidism in Cats

2011-05-21T10:57:00.008-04:00

Hyperthyroidism is one of the most common endocrine conditions affecting cats, especially older cats over the age of 10 years. Some veterinarians estimate that about 5 to 10% of middle- to old-aged cats will develop hyperthyroidism, and, due to factors that may include environmental exposures, that number is on the rise.

Thyroid hormones normally regulate many of the body’s metabolic processes. As circulating levels of thyroid hormones rise higher and higher in cats with hyperthyroidism, a progressive increase in their metabolic rate develops: this leads to a multitude of changes throughout the entire body, including loss of weight and muscle wasting. High thyroid hormone concentrations also interact with and stimulate the central nervous system, which commonly leads to increased activity or restlessness, as well as other behavior problems.

Clinical signs associated with hyperthyroidism can be quite dramatic and cats can become seriously ill with this condition. Untreated, hyperthyroidism in cats can lead to heart or kidney failure and can be fatal.

However, this disorder is a very treatable disease and most hyperthyroid cats will make a complete recovery. I’d discuss the various treatment options, together with pros and cons of each form of treatment, later in this current series of posts I’m doing on feline hyperthyroidism.

Do You Know the Clinical Signs of Hyperthyroidism to Watch for in Your Cat?
Here I’ve listed the top 10 most common signs that cats with hyperthyroidism can develop, from most common to least frequent.

Your cat doesn’t need to have all of these clinical signs or symptoms in order to have an overactive thyroid problem. But if your cat has one or more of these signs, especially if he or she is older than 10 years of age, you should definitely have your cat tested for hyperthyroidism.

1. Weight loss

Weight loss despite a normal to increased appetite is the classic and most common sign of hyperthyroidism in cats. Hyperthyroidism is so common that it should always be considered as a possibility in any middle-aged to older cats that has lost weight, even in none of the other clinical features of the disease are present.

The weight loss associated is generally progressive and is usually first noticed by the owner as a loss of muscle mass around the cat’s back (spine).

With time, severe muscle wasting, emaciation, and death from starvation can occur if the cat’s hyperthyroidism is left untreated.

2. Increased appetite
The weight loss characteristic of cats with hyperthyroidism is typically associated with an increase in the cat’s appetite. This increase in appetite can be dramatic, with some cats doubling the amount of food eaten and frequently begging for food. Hyperthyroid cats eat more in an attempt to compensate for their higher-than-normal metabolic rate by increasing the number of calories ingested. Unfortunately, most cats cannot fully compensate and continue to loss weight even if they have a good to increased appetite.

Although most hyperthyroid cats have an increased appetite, some cats with mild hyperthyroidism will maintain a normal appetite, with no obvious change in amount of food consumed.

A few hyperthyroid cats will even develop a reduced appetite that is improved after treatment of their hyperthyroidism. Most of these cats with a poor appetite have a rare form of hyperthyroidism called “apathetic hyperthyroidism,” which is described in more detail below (see clinical sign Number 10).

3. Hyperactive, increased energy, or nervous behavior
Hyperactivity, exhibited particularly as nervousness or restlessness, is relatively common in cats with hyperthyroidism. In extreme hyperthyroidism, muscle tremor or twitching may be apparent, and affected cats are often described as having an anxious or frantic facial expression.

In some cats, especially in the earlier stages of their hyperthyroidism, their owner can misconstrue such increased activity and energy as a positive sign of health. One must remember that cats normally become quite sedentary as they age so this change of behavior leading to increased energy and activity is almost always a manifestation of an underlying disease processes.

Some of these behavior signs — especially restlessness or aggression — are often more noticeable to the examining veterinarian than to the cat owners themselves. Many hyperthyroid cats have an impaired tolerance for stress and do not wish to be held or restrained. Some will even develop a “panic attack” as a result of an impaired tolerance for examination or restraint during blood sampling. These cats may also develop panting, overt respiratory distress, weakness, and even collapse during these episodes.

To see a video of severe anxiety in a cat with severe untreated hyperthyroidism, see my previous blog post.

4. Increased thirst and urination
About half of cats with hyperthyroidism will show signs of increased thirst (polydipsia) and urination (polyuria). Veterinarians often refer to polyuria and polydipsia simply as “PU/PD.”

There are a number of possible reasons for the increase in thirst and urination seen in hyperthyroid cats. The most common explanation for the PU/PD is mild, concurrent kidney disease. About a third of all cats older than 12 years of age will develop kidney disease, so the finding of concomitant kidney disease in a hyperthyroid cat should not be surprising.

Two other causes for polyuria and polydipsia in cats with hyperthyroidism include the following:

Increased kidney blood flow occurs secondary to hyperthyroidism, which can lead to what’s called “renal medullary washout.” This leads to the inability of the cat to concentration their urine normally.
Compulsive water drinking secondary to a behavior problem may also occur in some cats with hyperthyroidism.

In both groups of these cats, the increased thirst and urination return to normal after treatment of the cat’s hyperthyroidism.

5. Vomiting or regurgitation
Gastrointestinal signs including intermittent vomiting or regurgitation are fairly common in cats with hyperthyroidism. Vomiting may result from a direct action of thyroid hormones on an area of the brain called the chemoreceptor trigger zone. In other cats, the vomiting can result from gastric stasis (e.g., delay emptying of the stomach).

Vomiting appears to be more common in cats from multi-cat households and usually occurs shortly after feeding. Therefore, vomiting and regurgitation in most hyperthyroid cats may simply be related to rapid overeating. In other words, these cats eats too much food too fast, overfill their stomach, and then vomit.

One solution to this problem during the early stages of treatment is too feed the cat smaller amounts of food more frequently (every 3 to 4 hours, if possible).

6. Anxiety, night yowling, confusion, aimless pacing
The behavioral sign most obvious to owners is night yowling (a long loud mournful cry). This yowling most often occurs in the middle of the night for no apparent reason, and generally causes the owner to awaken. Aimless pacing and easily interrupted sleep patterns may also occur in some hyperthyroid cats.

All of these behavior signs appear to reflect a state of confusion, anxiety, or restless associated with the increased central nervous stimulation caused by the hyperthyroidism.

7. Diarrhea, soft stools, voluminous stools
Soft stools and diarrhea can occur in about a third of cats with hyperthyroidism. Other cats develop large voluminous stools with frequent defecation.

It is likely that accelerated gastrointestinal transit contributes to the increased frequency of defecation, soft stools and diarrhea. Some of these cats also develop malabsorption, in which they do not normally absorb the food from the intestinal tract.

8. Fast respiratory rate, panting, difficulty breathing
Respiratory abnormalities, including a rapid respiratory rate, panting, or difficulty in breathing at rest, are also common. Respiratory signs tend to occur most frequently during periods of stress. However, some affected cats will be noticeably intolerant of heat and seek out cooler places to sit, and some (especially advanced cases) may pant or breath more rapidly in warm or hot home environments.

In the absence of heart failure, weakness of the respiratory muscles due to chronic hyperthyroidism is the most probable reasons for these signs. However, central nervous or psychogenic effects also play a major role in development of these respiratory signs, especially in states of stress.

When severely stressed, some cats with hyperthyroidism will develop severe respiratory distress, rapid breathing, and panting; in addition, these cats may eventually become extreme weak or even collapse from exhaustion and shortness of breath. Hyperthyroid cats should therefore be handled carefully and in the veterinary office.

9. Skin, hair coat, and nail changes

Unkempt, matted hair coat

Skin and hair coat changes often develop in hyperthyroid cats. The hair coat, especially in long-haired breeds, is often unkempt, dull, and may even be matted.

Some hyperthyroid cats can groom obsessively resulting in alopecia (baldness) or a miliary dermatitis (crusty rash, often with intense itching). This is sometimes associated with an underlying skin allergy, but the skin problem is magnified by the cat’s apparent obsessive and compulsive behavior associated with increased licking and grooming.

Excessive nail growth may also occur, especially in cats with chronic and advanced hyperthyroidism. These nails appear thickened and may be more fragile that normal.

10. Apathetic hyperthyroidism
A small percentage (less than 5%) of hyperthyroid cats will show atypical signs where hyperexcitability or restlessness is replaced by depression, apathy, or weakness. Although weight loss is present in these cats, it is accompanied by poor appetite, instead of increased appetite. These disparate signs heighten the importance of clinicians having a high index of suspicion for such a common disease.

Many cats with apathetic hyperthyroidism will have concurrent severe non-thyroidal illness such as kidney failure, heart disease, or cancer. Therefore, it is wise to do a complete workup in these cats searching for secondary or concomitant disorders. Occasionally, however, no other medical problem can be identified and the cats respond completely to treatment of hyperthyroidism alone, with resolution of the depression and poor appetite.

In my next blog post on hyperthyroid, I’ll describe the 12 most common findings that your veterinarian may find when they perform a physical examination on a hyperthyroid cat.

Why Has Hyperthyroidism in Cats Reached Epidemic Levels?

2011-05-10T12:43:00.000-04:00

Hyperthyroidism is a very common disorder of older cats. Also called thyrotoxicosis, hyperthyroidism is caused by an increase in production of the two main thyroid hormones — known as thyroxine (T4) and triiodothyronine (T3) from an enlarged thyroid tumor.

Thyroid hormones affect nearly all the organs in the body; they play an important role in controlling the body's metabolic rate and thus the general activity level. This increased metabolic rate explains why hyperthyroid cats tend to burn up energy too rapidly and suffer weight loss despite having an increased appetite and food intake.

I first described the first cats with hyperthyroidism in 1979, only just over 30 years ago (1). Since then, hyperthyroidism has become the most common endocrine disorder in cats and a disease frequently diagnosed by all veterinarians in small animal practice. It is unclear why such a phenomenon has occurred.

Undoubtedly, increased awareness of hyperthyroidism by veterinary practitioners and by cat owners, easier availability of diagnostic tests, and a growing pet cat population have all played a role. However, its increased prevalence is not likely to be the result of an aging cat population alone. It may truly be a new disease that has just developed over the last three decades.

Incidence of feline hyperthyroidism varies around the world
The prevalence of hyperthyroidism varies geographically and at least anecdotally, regionally. It is a disease that is frequently diagnosed throughout North America, Europe, the United Kingdom, Australia, and New Zealand.

More recently, investigators in Japan have also reported an increasing number of cases. However, the disease appears to be much less common in other countries that have excellent veterinary care, such as in Spain and Hong Kong (2,3).

Thyroid gland changes associated with hyperthyroidism in cats
One of the largest endocrine gland in the body, the thyroid gland is found in the neck area, situated below the larynx (Adam's Apple) and adjacent to the trachea (windpipe).

In all species, including the cat, the thyroid gland is composed on 2 cone-like lobes or wings (in other words, there are 2 thyroid lobes that comprise the single thyroid gland). In most cats with hyperthyroidism, a benign or non-cancerous tumor (adenoma) develops in one or both of the thyroid lobes (4,5).

By contrast, thyroid cancer (carcinoma) is a rare cause of hyperthyroidism in cats, accounting for the tumors in less than 5% of hyperthyroid cats with hyperthyroidism (5,6).

But what’s causing these thyroid tumors to develop in the first place?
To date, the underlying factor(s) responsible for the thyroid changes remains obscure and is probably multifactorial. However, studies have indicated numerous environmental and nutritional risk factors, which may play a role in the why and how this disorder develops.

Possible risk factors:

Example of canned food
with pop-top lid

Diet composed entirely or primarily of canned cat food (7,9,15,18).
Certain varieties of canned cat good, such as fish, liver, or giblet flavor (7, 10,18).
Use of cat litter (7.9,18)
Diets containing either excess or deficient amounts of iodine have been implicated (8, 17,19)
Diets containing high amounts of selenium have also been suggested to have a role (11).
Cans with plastic linings and pop-top lids may pose a greater risk than sachets or cans, which require a can opener to open them (13,18). This is potentially due to the release of chemicals such as bisphenol-A and bisphenol-F from the lacquer linings of the pop-top cans.
Soy isoflavones (genistein and daidzein) are common constituents of commercially available cat foods and also may interfere with normal thyroid function (12,14).
Regular use of insecticidal products (flea products) on the cat or fly sprays within the household (7,9,15).
Exposure to herbicides and fertilizers (7,15).

Exposure to flame-retardant chemicals contaminants including polybrominated diphenyl ethers (PBDEs). Excessive PBDEs have been identified in household dust from contaminated carpet padding, polyurethane foams, furniture and mattresses. High levels of PBDEs have both identified in the serum of both dogs and cats, indicating that this in a some environment contaminant (16,20).

So what’s the bottom line?

Given the variety of abnormalities and associations described it is likely that hyperthyroidism is a multifactorial disease. In other words, more than one factor is contributing to its development and pathogenesis.

It is also important to realize, however, that many of the risk factors listed above also exist in areas of the world where hyperthyroidism is considered relatively uncommon. This emphasizes the complexity of the problem in identifying the cause(s) of hyperthyroidism in cats.

References:

Peterson ME, Johnson JG, Andrews LK. Spontaneous hyperthyroidism in the cat. Proceedings of the American College of Veterinary Internal Medicine 1979:p. 108.
Wakeling J, Melian C, A. F, et al. Evidence for differing incidences of feline hyperthyroidism in London, UK and Spain. Proceedings of the 15th ECVIM-CA Congress 2005;2005.
De Wet CS, Mooney CT, Thompson PN, et al. Prevalence of and risk factors for feline hyperthyroidism in Hong Kong. Journal of Feline Medicine and Surgery 2009;11:315-321.
Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645, v.
Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
Scarlett JM. Feline hyperthyroidism: A descriptive and case-control study. Preventive Veterinary Medicine 1988;6.
Tarttelin MF, Ford HC. Dietary iodine level and thyroid function in the cat. Journal of Nutrition 1994;124:2577S-2578S.
Kass PH, Peterson ME, Levy J, et al. Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1999;13:323-329.
Martin KM, Rossing MA, Ryland LM, et al. Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. Journal of the American Veterinary Medical Association 2000;217:853-856.
Foster DJ, Thoday KL, Arthur JR, et al. Selenium status of cats in four regions of the world and comparison with reported incidence of hyperthyroidism in cats in those regions. American Journal of Veterinary Research 2001;62:934-937.
Court MH, Freeman LM. Identification and concentration of soy isoflavones in commercial cat foods. American Journal of Veterinary Research 2002;63:181-185.
Edinboro CH, Scott-Moncrieff JC, Janovitz E, et al. Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. Journal of the American Veterinary Medical Association 2004;224:879-886.
White HL, Freeman LM, Mahony O, et al. Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats. American Journal of Veterinary Research 2004;65:586-591.
Olczak J, Jones BR, Pfeiffer DU, et al. Multivariate analysis of risk factors for feline hyperthyroidism in New Zealand. New Zealand Veterinary Journal 2005;53:53-58.
Dye JA, Venier M, Zhu L, et al. Elevated PBDE levels in pet cats: sentinels for humans? Environmental Science and Technology 2007;41:6350-6356.
Wakeling J, Elliott J, Petrie A, et al. Urinary iodide concentration in hyperthyroid cats. American Journal of Veterinary Research 2009;70:741-749.
Wakeling J, Everard A, Brodbelt D, et al. Risk factors for feline hyperthyroidism in the UK. Journal of Small Animal Practice 2009;50:406-414.
Edinboro CH, Scott-Moncrieff JC, Glickman LT. Feline hyperthyroidism: potential relationship with iodine supplement requirements of commercial cat foods. Journal of Feline Medicine Surgery 2010;12:672-679.
Venier M, Hites RA. Flame retardants in the serum of pet dogs and in their food. Environmental Science and Technology 2011.

Do All Hyperthyroid Cats Have a Thyroid Tumor? Is It Thyroid Cancer?

2011-05-07T17:06:00.000-04:00

Yes, all hyperthyroid cats have thyroid tumors – every last one. However, if your cat is hyperthyroid, there is no need to panic! Most of the thyroid tumors that cause hyperthyroidism in cats are benign (non-cancerous), so it is very unlikely that your cat has cancer, especially if the hyperthyroid condition was only recently diagnosed.

All thyroid tumors in hyperthyroid cats are made up of thyroid cells that grow and function at an uncontrolled rate. This makes your cat’s thyroid gland grow larger and larger with time. In almost all hyperthyroid cats, the veterinarian can palpate the thyroid tumor (Figure 1). In contrast, the veterinarian should not be able to feel a normal cat’s thyroid gland, much less see it!

Figure 1: A hyperthyroid cat with an extremely large thyroid tumor.

In addition to making the thyroid larger, the tumor cells function at an increased rate and produce too much of the thyroid hormone thyroxine (T4). This excess thyroid hormone in your cat's circulation is what causes the signs of hyperthyroidism, such as increased appetite, hyperactivity, and weight loss.

Thyroid tumors come in two varieties, adenoma and carcinoma. Adenomas are by far the majority (97-98%) of tumors that cause hyperthyroidism in cats. Adenomas are benign tumors; they do not spread (metastasize) to other parts of the body. The normal feline thyroid gland is made up of 2 small, separate thyroid lobes, both lying adjacent to the trachea (windpipe). In 70% of hyperthyroid cats, adenoma infiltrates both thyroid lobes (Figure 2), whereas 30% of cats have only one thyroid lobe affected.

Figure 2: Hyperthyroid cat with two thyroid adenomas at time of surgery. In this cat, the left thyroid lobe (top of image) is twice normal size, whereas the right lobe (bottom of image) is 4-6 times normal size. Both thyroid lobes were palpable on physical examination.

In 2-3% of cats, the hyperthyroid condition is caused by thyroid carcinoma, rather than thyroid adenoma. Unlike adenomas, thyroid carcinomas are cancerous and generally grow to a massive size (Figure 3). They are malignant tumors – carcinomas invade and metastasize to surrounding tissues, such as the chest cavity, and endanger the cat’s life.

Figure 3: Hyperthyroid cat a large thyroid carcinoma at time of surgery. In this cat, the left thyroid lobe is approximately 50 times normal size. These tumors are invasive and generally inoperable.

Although cats with thyroid carcinomas are harder to treat, radioiodine therapy is the best option. By administering an ultra-high dose of radioiodine (e.g. up to 10 times the I-131 dose appropriate for an adenoma), it is easily possible to destroy all of the cat’s malignant thyroid tissue. With this treatment, we can cure the cat of both their cancer and hyperthyroidism!

One question remains: what causes the 2 to 3% of hyperthyroid cats to develop carcinomas? Again, most cats that have carcinomas have been hyperthyroid for a long time (on the scale of many months to years). Recent evidence indicates that the tumors change over time in these cats, transitioning from normal tissue to fast growing tissue, soon turning into an adenoma. When left untreated, or treated only with an anti-thyroid drug (see next week’s article), tumor growth continues and eventually the adenoma tissue transforms into a malignant carcinoma.