Transitioning Hyperthyroid Cats from Methimazole to Hill’s y/d: If It Ain’t Broke, Don't Fix It!

My patient is a 15-year old, F/S, DSH cat with a 2-year history of hyperthyroidism. Her thyroid values had been very well controlled on methimazole. However, she was becoming a “pain” to pill daily, and her methimazole dosage had recently increased from 2.5 mg to 5.0 mg per day. She has a past history of diabetes and had been in remission for 3 years on a low-carbohydrate, high-protein diet (dry Purina DM, ½ cup per day). She is a dry food addict and refuses to eat any canned cat food.

When Hill’s y/d came out, I thought that it was the perfect solution for this cat. Although she didn’t appear to like the y/d during the first week, she was eating it fairly well by the second week. Her methimazole dosage was completely phased out over the course of two weeks. I rechecked her serum T4 concentration at 4 and 8 weeks after starting the y/d and her T4 values were 3.9 μg/dl and 3.5 μg/dl, respectively (both in the upper end of the reference range).

My concern is that this cat has become ravenous, constantly begging for more food. The owner has fed the dry y/d in the amounts recommended by Hill’s (½ cup per day) and has even increased the amount fed to the cat. However, the cat doesn’t always finish the y/d she is fed and still cries for more food (obviously, she wants to eats something else).

The cat is starting to look quite emaciated, losing 2 pounds and a large amount of muscle mass over the 8 weeks she has been on the y/d diet. Her last serum glucose concentration was also higher than I’d like (at 210 mg/dl), although not high enough for me to diagnose overt diabetes and start insulin treatment once again.

Because of the cat’s clinical response, I decided to take her off the y/d and restart the methimazole and her regular food. She has only been back on the Purina DM for two weeks but already looks much better and has regained a pound. She certainly is acting much happier and is more content. The constant begging and crying have stopped. Both the serum concentrations of T4 (2.2 μg/dl) and glucose (125 mg/dl) are lower than they were on the y/d.

Is this a problem Hill's is seeing with the y/d? I feel like I did more harm than good to my cat by switching her over from methimazole to the y/d. What are your thoughts on this food?

My Response

In the future, you may want to follow the advice of the adage (1), "If it ain’t broke, don't fix it!”

Let me first state that most cats that are treated with Hill’s y/d diet respond better than your cat did (it would be difficult to have a worse response!). But this case does bring up the issue that many cats will not respond completely to this diet, and some cats, such as this patient, will completely fail to respond.

And remember that even if a cat does respond adequately, the diet must be continued as the sole food source for the rest of the cat’s life to maintain euthyroidism (2). With y/d, the underlying cause of a cat’s hyperthyroidism will never be cured; rather, the hyperfunctional thyroid tumor(s) remains and may continue to grow larger with time (3,4).

So what happened to your cat to cause the failure of y/d? Well, it looks like a number of issues contributed to the problems with rapid weight loss, muscle wasting, and recurrence of hyperglycemia that this cat experienced while being fed Hill’s y/d diet.

Poorly controlled hyperthyroidism on the y/d diet
First of all, the serum T4 concentration in this hyperthyroid cat only decreased into the high-normal range. If you had measured a free T4 concentration in this cat when fed the y/d, I can guarantee that the free T4 value would have been quite high. No matter what treatment is given, it’s important to lower the total T4 concentration into the mid-normal range to ensure that the cat’s free T4 is also normalized and whole-body euthyroidism is restored (3,5,6).

So persistent and poorly controlled hyperthyroidism certainly contributed to the worsening of this cat’s weight loss and muscle wasting, despite the increased appetite in this cat. Remember hyperthyroidism accelerates the metabolic rate and body’s energy expenditure (7,8); these cats are burning up their food calories faster than they can consume their daily meals (5,6,9). In some cats, as in your patient, the body consumes its own muscle tissue to get the protein it needs to sustain its carnivorous life.

Decreased caloric intake on the y/d diet
In addition to the poorly controlled hyperthyroidism, it also appears that your cat was eating significantly fewer calories when fed the y/d. If we compare the caloric content of Purina DM to Hill’s y/d, we find that ½ cup of DM provides almost 300 kcal whereas ½ cup of y/d provides only 259 kcal (2,10). This 40 kcal per day might not seem like much, but that’s a 15% decrease in the amount of daily calories ingested!

Another important point in this cat is the fact that she did not find the y/d particularly palatable, and therefore did not eat more of the y/d food to compensate for the lower-caloric food. Remember, a few cats may actually like the y/d, but almost all of the cats I have seen on the diet would much rather eat another type of food if given the choice (11).

Decreased protein intake on y/d diet
In addition to caloric density, Hill’s y/d is also much lower in protein compared to the Purina DM (36% vs 58% on a dry matter basis) (2,10).

Remember that 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.

The lower protein intake of Hill’s y/d, together with the lower caloric intake, both contributed to the muscle breakdown and rapid loss of muscle mass in this cat when fed the diet. If this cat had been able to increase the daily amounts of y/d ingested, this may have lessened or prevented the loss of muscle mass that developed in the cat (12,13).

Higher carbohydrate intake on y/d diet
It is well accepted by most practicing veterinarians who specialize in feline medicine that a low-carbohydrate diet is a mainstay in the treatment of diabetes mellitus, especially if remission of the diabetic state is the goal. Feeding a low-carbohydrate diet will improve insulin sensitivity, reduce or eliminate the need for exogenous insulin, and help stabilize glucose metabolism in these cats (14-16).

If remission is achieved, it is very important to continue the low carb diet. In this cat, remission of diabetes was maintained for years by feeding Purina DM diet, which is relatively low in carbohydrates (i.e., 15% on a dry matter basis). Most feline veterinarians like to feed even lower levels of carbs (< 10%) to cats with active diabetes (14,15). To achieve these low carbohydrate levels, we must feed a canned food —none of the available dry cat foods contain <10% carbohydrates. If the diabetic cat goes into remission, we recommend maintaining a restricted carbohydrate diet for life to help prevent relapse of the diabetic state.

In contrast, Hill’s dry y/d diet is relatively high in carbs (i.e., 29% on a dry matter basis), much higher than the 15% found in Purina dry DM diet (2,10). This increased carbohydrate load likely contributed to this cat’s hyperglycemia and probably would have led to recurrence of overt diabetes mellitus if the y/d diet had been continued.

Hill’s y/d and the hyperthyroid cat: my bottom line

Overall, this case illustrates a number of potential problems with y/d for managing cats with hyperthyroidism.

1. First of all, the diet may not be totally effective in lowering serum total and free T4 values.
2. Secondly, the diet is not very palatable to many cats, and some cats either refuse to eat the diet or fail to eat enough.
3. Thirdly, because the y/d diet is relatively low in protein, weight loss and muscle wasting may result if the hyperthyroid cats fail to ingest adequate amounts of the diet to provide the calories and protein they need to restore normal body weight and muscle mass.
4. Finally, because y/d is a high-carbohydrate diet, this is certainly the worst type of diet to feed to a diabetic cat or a cat in diabetic remission (14-16). These cats should be ideally be fed a low carb canned diet, with higher levels of protein or fat.

For those of you who have read my other posts concerning the Hill’s y/d diet, you know that I’m not a big fan. Of course, we all care for hyperthyroid cats in which no other treatment option is found acceptable for the owner. But most cats, at least in the long-term, will do better with our other treatment options, especially definitive therapy that really addresses the cause — that is, the thyroid adenoma or carcinoma causing the hyperthyroid state.

At least for owners who are willing to do the best for their cat, isn’t it most logical to cure the thyroid disease rather than trying to control it with a diet that is too low in protein, too high in carbohydrates, and chock-full of grains that a cat would never eat in the wild? Would you really want your own cat to eat this diet when the long-term consequences are still not even known?

References:

1. Wikipedia.org. Bert Lance. Quote in Nation’s Business, May 1977. 
2. Hill's Pet Nutrition website. Prescription Diet y/d Thyroid Feline Health.
3. Insights into Veterinary Endocrinology blog. Treating Hyperthyroid Cats with an Iodine Deficient Diet (Hill's y/d): Does It Really Work? September 24, 2011. 
4. Peterson ME, Broome MR: Thyroid scintigraphy findings in 917 Cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; in press.
5. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney CT, Peterson ME (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association 2012; 92-110.
6. Baral RM, Peterson ME. Thyroid gland disorders: Hyperthyroidism & hypothyroidism. In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders 2012:571-592.
7. Morrison WL, Gibson JN, Jung RT, et al. Skeletal muscle and whole body protein turnover in thyroid disease. European Journal of Clinical Investigation 1988;18:62–68.
8. 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.
9. Little SE: Evaluation of the senior cat with weight loss. In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders 2012:1176-1181.
10. Purina Veterinary Diets website. DM Dietetic Management Feline Formula. 
11. Peterson ME.  Anorexia in a hyperthyroid cat on Hill’s y/d. Insights into Veterinary Endocrinology blog. February 7, 2012.  
12. 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.
13. Peterson ME. Can Increasing the Amount of Fat or Carbohydrate in a Cat's Diet Compensate for Low Protein Intake? Insights into Veterinary Endocrinology blog. December 22, 2011. 
14. 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.
15. Rand JS. Feline diabetes mellitus. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology, Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association 2012:133-147.
16. Baral RM, Little SE. Endocrine pancreatic disorders: Diabetes mellitus, gastrinoma, and insulinoma. In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders 2012:547-571.