Treating Cats with Hyperthyroidism: Antithyroid Drugs

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 overly 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

1. 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.
2. Baral R, Peterson ME. Thyroid diseases. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; in press.
3. 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.
4. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
5. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157. 
6. Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. Journal of Small Animal Practice 1992;33:228–235. 
7. 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.
8. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
9. 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.
10. 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. 
11. 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. 
12. Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
13. Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355. 
14. Plumb DC. Plumb's Veterinary Drug Handbook (7th Ed). PharmaVet Inc, Stockholm, Wisconsin 2011.
15. Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy. Australian Veterinary Journal 2000;78:462-465. 
16. 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.
17. 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. 
18. 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. 
19. 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. 
20. Buijtels JJ, Kurvers IA, Galac S et al. Transdermal carbimazole for the treatment of feline hyperthyroidism, Tijdschrift voor Diergeneeskunde 2006;131:478-482. 
21. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
22. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
23. 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. 
24. 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. 
25. 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. 
26. 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. 
27. 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.
28. 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. 
29. 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.  
30. 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. 
31. 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. 
32. 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 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: 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

1. 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. 
2. 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. 
3. Baral R, Peterson ME: Thyroid Diseases, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-692 
4. 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. 
5. 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. 
6. Hoenig M, Ferguson DC. Impairment of glucose tolerance in hyperthyroid cats. Journal of Endocrinology 1989;121:249-251. 
7. Hoenig M, Peterson ME, Ferguson DC. Glucose tolerance and insulin secretion in spontaneously hyperthyroid cats. Research in Veterinary Science 1992;53:338-341. 
8. Short KR, Nair KS. Mechanisms of sarcopenia of aging. Journal of Endocrinological Investigation 1999;22(5 Suppl):95-105. 
9. Fujita S, Volpi E. Nutrition and sarcopenia of ageing. Nutrition Research Reviews 2004;17:69-76. 
10. 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. 
11. 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. 
12. 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. 
13. 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. 
14. 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. 
15. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42. 
16. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562. 
17. Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutrition Research Reviews 2002;15:153-168. 
18. Zaghini G, Biagi G. Nutritional peculiarities and diet palatability in the cat. Veterinary Research Communications 2005;29, Suppl 2:39-44 
19. Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association 2002;221:1559-1567. 
20. 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. 
21. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178. 
22. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9. 
23. 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. 
24. 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. 
25. 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. 
26. 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. 
27. Plotnick A. Feline chronic renal failure: Long-term medical management. Compendium for Continuing Education for the Practicing Veterinarian 2007;29:342-324, 346-350. 
28. Kidder AC, Chew D. Treatment options for hyperphosphatemia in feline CKD: what's out there? Journal of Feline Medicine and 2009;11:913-924. 
29. 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. 
30. 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. 
31. Laflamme DP. Determining metabolizable energy content in commercial pet foods. Journal of Animal Physiolology and Animal Nutrition (Berlin). 2001; 85:222-230. 
32. Thompson A. Ingredients: where pet food starts. Topics in Companion Animal Medicine 2008;23:127-132. 
33. FDA. Selecting nutritious pet foods.  
34. Remillard RL. Homemade diets: attributes, pitfalls, and a call for action. Topics in Companion Animal Medicine 2008;23:137-142. 
35. Schenck PA. Home-Prepared Dog and Cat Diets. Second Edition. Wiley-Blackwell, Ames, Iowa. 2010.
36. FDA. Is it safe for me to provide my pet with a raw food diet?
37. 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.
38. Feline Nutrition Education Society. Just what is raw feeding, anyway?
39. Pierson LA, CatInfo.org. Making cat food.

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  

Daily Water Requirements and Needs for Cats

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

1. 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.
2. 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.
3. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
4. 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.
5. Angilletta MJ. Estimating body composition of lizards from total body electrical conductivity and total body water. Copeia 1999;3:587–595.
6. Austad S, Kristan D. Does caloric restriction of laboratory mice mimic natural food intake of wild mice? Gerontology 2002;42:8–18.
7. 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. 
8. 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.
9. 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. 
10. 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
11. Adolph EF. Tolerance to heat and dehydration in several species of mammals. American Journal of Physiology 1947;151:564–575. 
12. Seefeldt SL, Chapman TE. Body water content and turnover in cats fed dry and canned rations. Am J Vet Res 1979;40:183–185. 
13. 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.
14. Food and Drug Administration website. Pet food labels—general.