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

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:

1. 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
2. Taubes G: Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease 2007.
3. Lustig RH. The fructose epidemic. The Bariatrician 2009:10-18. 
4. 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. 
5. 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. 
6. Petfood Industry (www.petfoodindusty.com), December 13, 2010. Sugar in pet treats may be contributing to obesity. 
7. The Dog Food Project (www.dogfoodproject.com). Ingredients to avoid.
8. 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.

Sugar: The Bitter Truth by Robert Lustig, MD

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 

1. 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. 
2. Lustig RH. Which comes first? The obesity or the insulin? The behavior or the biochemistry? The Journal of Pediatrics 2008;152:601-602. 
3. 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. 
4. Lustig RH. The fructose epidemic. The Bariatrician 2009:10-18. 
5. 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. 
6. Lustig RH. Fructose: metabolic, hedonic, and societal parallels with ethanol. Journal of the American Dietetic Association 2010;110:1307-1321. 
7. Nguyen S, Lustig RH. Just a spoonful of sugar helps the blood pressure go up. Expert Review of Cardiovascular Therapy 2010;8:1497-1499. 
8. 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:

1. 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.  
2. 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. 
3.  Petfood Industry (www.petfoodindusty.com), December 13, 2010. Sugar in pet treats may be contributing to obesity.  
4. The Dog Food Project (www.dogfoodproject.com). Ingredients to avoid.
5. 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

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).

Daily Water and Fluid Requirements and Needs for Hyperthyroid Cats

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 

1. 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. 
2. 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. 
3. 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. 
4. 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. 
5. 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. 
6. 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. 
7. Little S. Managing the senior cat. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, in press. 
8. 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. 
9. Radcliffe CE: Observations on the relationship of the thyroid to the polyuria of experimental diabetes insipidus. Endocrinology 1943;32:415-421. 
10. 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.
11. Evered DC, Hayter CJ, Surveyor I. Primary polydipsia in thyrotoxicosis. Metabolism 1972;21:393-404. 
12. 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. 
13. 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.
14. Is Bottled Water Better? Medicine.Net.com  

How is Canine Hypothyroidism Diagnosed?

Even though hypothyroidism is the most frequently recognized canine endocrine disorder in dogs, it is still difficult to make a definitive diagnosis of the condition. Since the thyroid gland regulates metabolism of all of the body’s cellular functions, reduced thyroid function can produce a wide range of clinical signs (see our last blog post). Many of these signs mimic those of other disorders and illnesses, making recognition of a thyroid condition and proper interpretation of thyroid function tests confusing and problematic for veterinarians.

The dog’s signalment can be important!

The first step in diagnosis of hypothyroidism is to review the dog’s signalment (i.e., age and breed). Most hypothyroid dogs are young adults, and certain breeds are predisposed to developing the condition (see our last last blog post on this topic). That said, we can see hypothyroidism in any breed of dog, and very young or old dogs may also be affected.

Dog’s history, clinical signs, and physical examination findings

Another part of how we diagnose is a review of the history, and clinical symptoms the dog with suspected hypothyroidism is exhibiting. The most common clinical features include mental dullness or lethargy, weight gain, and skin changes. (See our last blog post for a list of clinical signs and images of affected dogs)

General laboratory evaluation

All dogs with suspected hypothyroidism should have a general laboratory panel done prior to determination of any specific thyroid function tests. Ideally, this should include a complete blood count (CBC), comprehensive serum chemistry panel, and complete urinalysis.

• Complete blood count (CBC) – It is not uncommon for hypothyroid dogs to be anemic (normocytic, normochromic, nonregenerative anemia).

• Serum chemistry profile – Hypothyroid dogs have decreased fat metabolism and commonly have high levels of blood cholesterol (hypercholesterolemia) and lipids (hyperlipidemia).

• Urinalysis – A complete urine analysis is important for assessing whether other diseases are affecting your dog either in addition to, or instead of hypothyroidism. This test is normal in hypothyroid dogs.

Although we frequently see changes in the general laboratory work that point to hypothyroidism as a cause of the dog’s clinical signs, the most important reason that we must do these test in to rule out other diseases that may mimic hypothyroidism. Many common non-thyroid diseases (e.g., diabetes, Cushing’s syndrome, kidney or heart disease, severe infections) can also cause circulating thyroid hormone levels in the blood to decrease.

Your veterinarian must perform these general lab tests to rule these diseases out before confirming a diagnosis of hypothyroidism.

Once these general laboratory tests have been done, the next step in diagnosis is to run one or more specific thyroid tests. That will be the topic of my next blog post.

What is T4? How is the T4 Test Used to Diagnose Hyperthyroidism in my Cat?

What is being tested?

This test measures the amount of thyroxine, more commonly referred to as T4 or total T4, in your cat’s blood. T4 is one of two major iodine-containing hormones produced by the thyroid gland (the other is called triiodothyronine, or T3). The thyroid is a small, bi-lobed gland located just below the Adam's apple. This gland plays a vital role in controlling the rate at which the body uses energy or controls metabolism. Important components of the T4 and T3 molecules include the amino acid tyrosine and iodine (see Figure 1).

Figure 1

T4 makes up nearly 90% of the circulating thyroid hormones, while T3 makes up less than 10%. Inside the thyroid gland, T4 is bound to a protein called thyroglobulin. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound) or protein-bound (primarily bound to serum protein). The concentration of free T4 is less than 0.1% of that of total T4. T4 is converted into T3 in the liver or other tissues. T3, like T4, is also highly protein-bound, but it is the free forms of T3 and T4 that are biologically active.

Although T3 is an important thyroid hormone, it is of little use in the diagnosis of feline hyperthyroidism; therefore, veterinarians rarely order this test.

If the thyroid gland produces too much T4, the rate of the cat’s body functions will increase and cause symptoms associated with hyperthyroidism such as weight loss, increased appetite, increased heart rate, anxiety, and vomiting. The most common cause of hyperthyroidism in cats is a benign thyroid adenoma but some cats will develop thyroid cancer or carcinoma. The effect of these conditions on thyroid hormone production can be detected by measuring the total T4 (includes both the protein-bound and free portions of T4) or the free T4 (measures just the free or unbound portion of T4).

How is it used?

Total T4 and free T4 are two separate tests that can help the veterinarian evaluate thyroid function. The total T4 test has been used for many years as a diagnostic test for humans, as well as dogs and cats. It is a very useful test and is the main test used to confirm suspected hyperthyroidism in cats.

What does the test result mean?

In general, a high total or free T4 result indicates an overactive thyroid gland (hyperthyroidism). The test results alone are not diagnostic but must be interpreted together with your cat’s clinical signs, symptoms, and physical examination findings.  

Since free T4 is the active form of thyroxine, it could be thought to be a more accurate reflection of thyroid hormone function than total T4. Indeed, the free T4 test is a more sensitive diagnostic test than is the T4 test, i.e., the free T4 test will be diagnostic in 98.5% of hyperthyroid cats whereas the total T4 test will be diagnostic in 91% of cats with the disease (See Figures 2 and 3). However, many cats suffering from other illnesses NOT associated with hyperthyroidism will have false-positive results with the free T4 test (Figure 3). For that 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.

Figure 2

Serum T4 results in 917 cats with hyperthyroidism, 221 cats with nonthyroidal disease (sick cats without hyperthyroidism), and 172 normal cats. Data are plotted as box plots, where the T‑bars represent the main body of data and the box represents the middle half of the data. The horizontal bar in the box represents the median. Outlying data points are represented by open circles. The shaded area indicates the reference range. Notice that none of the cats with nonthyroid illness have a high T4 value. From: Peterson ME, Melián C, Nichols CE: 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. 218: 529-536, 2001.

Figure 3

Serum free T4 results in 917 cats with hyperthyroidism, 221 cats with nonthyroidal disease (sick cats without hyperthyroidism), and 172 normal cats. Notice that many of the cats with nonthyroid illness have a high T4 value even though they are not hyperthyroid (false-positive result).