Monday, April 30, 2012

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?

  1. 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.


Kimberly Belk said...

Thank you for your comments on the Hills y/d. While I applaud this corporation's support of nutritional education in vet schools, I am not impressed AT ALL with their diets. My 15+ y/o female cat was diagnosed with DM in 10/11. The vet prescribed 2 units of Lantus insulin twice daily and Hills w/d. After a week, she still had sugars in the 4-500 range. I took it upon myself to put her on a raw diet (I'm a RN) and within 6 weeks was completely weaned off insulin and has had sugars ranging from 70-126 ever since. Today, a 3/6 murmur at the RSB was noted along with 4+ bilirubin in the absence of abnormal enzymes. She developed a UTI last week and was put on Clavimox; after 9 doses began throwing up and not wanting to eat, hence the visit to the vet today. The question of hyperthyroidism was raised and the suggestion of Hills y/d. The thyroid tests are not available yet, but does this sound like hyperthyroidism to you? Thank you in advance for your reply!

Dr. Mark E. Peterson said...

In a 15-year cat, hyperthyroidism is a very common differential. That said, most hyperthyroid cats show weight loss despite a good appetite. So her anorexia doesn't go along with typical hyperthyroidism but that may all be due to the antibiotic!

Remember that all hyperthyroid cats have a thyroid tumor so your vet hopefully would be able to palpate the goiter, if present.

suzanjane said...

My 15 year old has been on tapazole for over 2years... Earlier this year She was put into a clinical trial of hills y/ d and rapidly lost weight down to 3pounds.... I had her removed from the trial... She is back on Tap. I have been feeding her a high protein diet w canned food (98 % meat) with some dry food (Origen).. But she is always looking for more meat... Her stools are extremely 'plentiful' soft and horrifically smelly! I am at my wits end about what to feed her.. I had also cooked chicken and added rice all to no avail.... She will not eat raw.... I feel she isn't tolerating the high protein diet but don't know what other options there are. Any suggestions would be helpful.... This is a very distressing condition for both of us. Thank you

Dr. Mark E. Peterson said...

It sounds like your cat may have a primary GI problem. I'd recommend that you consult with Dr. Lisa Pierson (check out her website at She may be able to formulate a good diet for you that will help your cat. Good luck!

Fred B said...

Thanks Dr. Peterson, GI was the suspecion when I went to the vet and I have talked with Dr Lisa Pierson. I've tried all the ususal things, forti-flora, pro-pectin, digestive enzymes, many homeopathic remedys that made him vomit! As much as hate Hill's foods and their huge amount of carbs, the current hydralized protein seems to be helping. My other cat, a Bengal, was severely overweight, 20+ lbs free feeding hills. I got her down to 17 lbs and went as low as about 150 cal a day (I worry about fatty liver disease going lower) but while she's still a big cat, will not go any lower. I think her ideal weight is probably 14-15 lbs. I even built her a 4 foot cat wheel. She eats wellness wet and some dry but it's tough to convince her to eat the wet food anymore. None of our cats feed feed anymore if they insist on dry it's either EVO or Nature's Variety low carb. Thanks again.

Sandi Baird said...

My 15 year old male Siamese cat was recently diagnosed as hyperthyroid. My vet put him on Hills, dry, y/d. At first, he ate the food, but then slowly began refusing to eat it. He lost three pounds within about two months, down from 12 to 9 pounds. He also began losing teeth. My vet said this is normal, a sign of age? This week, at my request, my vet prescribed Methimazole tablets and I have ceased the y/d. So far, his appetite has returned and he seems much happier. All in a matter of a few days. I was unaware of the protein issue with y/d until I read the posts on your blog, Dr. Peterson. And, I was unaware that y/d was only a bandaid. Thank you.

Dr. Mark E. Peterson said...

Unfortunately, many cats that I see on the y/d diet will lose weight, both because they aren't eating enough food (it doesn't taste very good!) and as the result of loss of muscle mass.

I'm happy to hear that the situation is improved now that you have stopped the diet!

Natalie Smyth said...

My elderly cat was recently diagnosed with multiple health problems(pancreatitis, hyperthyroidism and hepatic lipidosis) before visiting the vet she was losing weight and eating non stop but vomiting every few days. She is a small cat but went from being 8 pounds to 6 pounds in a few months. The vet put her on the Y/D diet. She will not eat it but does seem interested in her regular food. My fear is that she will die from malnutrition. I'm wondering if there is any other diet plan that will help my cat gain weight again but help with the hyperthyroidism?

Thank you

Dr. Mark E. Peterson said...

If your cat will not eat the y/d (and this is a very common problem!), then nutritional therapy for hyperthyroidism using a low iodine diet isn't going to work.

If your cat is loosing body weight and muscle mass to start with and then isn't eating because she doesn't like the y/d diet, this will only make her problems worse.

I'd change to another higher protein diet that your cat will eat. I'd try methimazole treatment for the hyperthyroidism; because of the multiple other problems, using the transdermal route for treatment may be best for her. But talk to your vet about this and other treatment options.

suzanjane said...

My6 year old female has been on transdermal methimazole for over 4 years; at one point she was in a y/d trial and lost 2 of her 5 pounds in 2 weeks! i have her on a high protein dry food as well as a meat wet food. she still weighs just over 4 pounds but is quite frisky. the only concern i have is that she seems to be developing resistance to the meds and may need to have even more each day than currently.