Monday, October 17, 2011

Diet and Nutritional Management of Hyperthyroid Cats

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:

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 (, 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:
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., 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.

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addypotter said...

Thank you for such an informative post. My cat was diagnosed with hyperthyroidism last week. My vet told us that there is a drug methimazole for cats out there that should help. Have you ever used it? What are the risks/benefits?

Dr. Mark E. Peterson said...

You veterinarian is talking about the new diet y/d, which is an iodine deficient diet designed to limit the amount of thyroid hormone that a cat can make.

I'm not a big fan of the diet, and I'm worried about long term side effects. To learn more on my thoughts, check out my posts about y/d at my other blog at and search for y/d.

Renee Michelle said...

Thank you for your blog! What are the best carbohydrate sources for a cat? For example, rice, flaxseed? Other? I am feeding my cats a reconstituted raw diet with bone and ground pumpkin seed that is enriched with probiotics and krill oil. They are doing well and my hyperthyroid guy is slowly gaining muscle mass/weight.

Dr. Mark E. Peterson said...

Cats are true carnivores and have absolutely no requirements for any type of carbohydrate. So in my opinion, there aren't any "best" types of carbohydrates since none are needed.

If possible, I'd recommend trying to keep the percent carbohydrates (dry matter basis) to less than 10-15%. That means that we will have to feed more protein and fat, which is what cats are meant to eat.

CAR11 said...

My cat has just been diagnosed with hyperthyroidism and results also show "moderate" kidney disease. He has advised the y/d diet because the medication can worsen the kidney disease? Problem is that I have 2 house cats and separating the feed is a real problem. Also I have read in a number of posts that y/d is really not the way to go. Is there a food/diet that would be suitable for both cats? I am also getting really confused as to the best route to take for hyperthyroidism, particularly as kidney disease has also been identified.

Dr. Mark E. Peterson said...

I would never advice feeding y/d to a normal cat, especially since low iodine is one known potentiating cause for hyperthyroidism, both in man and cats. It's also too low in protein for normal cats.

Hill's y/d is a good renal diet, but I don't know what "moderate" means. If the serum creatinine is above 3 mg/dl, then a renal diet should be started.

Sounds like you need to have another talk with your veterinarian to determine what's the best course of treatment. Good luck!

Unknown said...

Thank you for the information. I just clicked through on the link you provided for the food percentage breakdowns and clicked on the wet food and it took me to a spam site about millionaires. Do you have the correct link or can you please suggest which food I should feed my 9 yr old female hyperthyroid cat? I started her on y/d 5 days ago and she is very anxious and seems uncomfortable. Your suggestions would be greatly appreciated.
Thank you!

Dr. Mark E. Peterson said...

Go to