Should Hill's y/d Diet Ever Be Used to Manage Cats with Hyperthyroidism?

Hyperthyroid cat with an allergic reaction (facial pruitus) secondary to methimazole

We are unlikely enough to have not one... but two cats with hyperthyroidism. One has responded well to treatment with methimazole, with normalization of the serum T4 value, while the other cat had a severe allergic reaction (facial pruitus) so we had to stop the medication. Because of her age and how ravaged she was by the allergic reaction, we didn't want to do radiation or surgery.

My veterinarians suggested that we could try Hills' y/d diet, since no other good options were available. Since we had no other options available, we felt that we had nothing to lose.  Luckily, our cat will eat the diet. Slowly, she has improved and is doing fairly well.

Who knows if this food will be good for her long term (I take your comments seriously about what cats truly need in their diet), but she's almost 16-years old and has enjoyed a good life. Considering our situation, trying the diet was worth a shot, and it has lowered the serum T4 value to normal.

Now I'm feeling guilty — should I continue the y/d or should I rethink my decision not to use the radioiodine?

My Response:

I consider the feeding of Hill's y/d diet to be a fourth-line of treatment for hyperthyroidism; I'd recommend this treatment only if none of the 3 other options (i.e., radioiodine, surgical thyroidectomy, or methimazole) can be used (1-3).

If a hyperthyroid cat will eat this ultra-low iodine food (and not eat anything else), the y/d certainly will lower the serum thyroid hormone concentrations (4). Remember that iodine is an essential nutrient and is needed to make thyroid hormone; with this diet, we are inducing an iodine deficient state so the cat's thyroid tumor will not be able to continue to produce large amounts of thyroid hormone on this diet (5).

However, like methimazole, feeding an ultra-low iodine diet can not cure the primary cause of hyperthyroidism (ie, the thyroid tumor). The thyroid tumor, which is almost always benign at time of diagnosis) will continue to grow with time (5,6). In some cats, this benign tumor can also transform to a more malignant carcinoma after a few months to years. So whenever possible,  I believe it's always better to remove the thyroid tumor with surgery or ablate it with radioiodine —in that way, we are curing rather than prolonging the disease and not allowing the thyroid tumor to continue to grow.

Now, if you consider 16 year's of age too old for definite treatment, then y/d can certainly be fed. For a geriatric cat, I don't consider 15 or 16 years of age too old for definitive treatment unless other concurrent problems are present that are known to shorten their lives. Remember that old age itself isn't a disease, and I see many senior cats that are still doing well at 18 to 20 years of age.

The main advantage of the methimazole over the y/d is that we can control what the cat is fed (remember that cats managed with y/d can ONLY eat that diet).  The best diet for a hyperthyroid cat or any senior cat for that matter is one that is lower in carbohydrates (<15% of calories) and higher in protein (>35-40% of calories) (7).  Senior cats don't absorb protein as well as younger cats and will develop weight loss and muscle wasting as they age (8-11). Other than exercise (and good luck with getting most cats to enter a weight lifting regime), the best way we know to maintain muscle mass is to feed a higher protein diet.

When we look at the dietary composition of Hill's y/d, it's way too high in carbs (>23% of calories), and much too low in protein (<28% of calories). In addition, the protein that's contained in the dry y/d is all plant protein, which isn't as good as animal protein is for cats — when was the last time you heard of a feral cat that was a vegetarian?

Obviously, in your cat, we can never use methimazole again because of the severe allergic reaction that occurred.  So, should you continue the y/d for your cat?  That's a decision that you will have to make— remember that any treatment, including the y/d, is better than no treatment at all.

But I like to treat the whole animal, however, and that involves more than just giving pills, feeding an iodine-deficient diet, or even giving an injection of radioiodine. For me, that involves proper geriatric nutrition, supplements in some cats, and finally, environmental stimulation to help enrich their lives (1-3, 7).

References: 

1. Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association. 2012;92-110.
2. Baral RM, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders, 2012;571-592.
3. Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
4. Yu S, Wedekind KJ, Burris PA, et al. Controlled level of dietary iodine normalizes serum total thyroxine in cats with naturally occurring hyperthyroidism [abstract]. J Vet Intern Med 2011;25:683-684.
5. Peterson M. Hyperthyroidism in cats: What's causing this epidemic of thyroid disease and can we prevent it? J Feline Med Surg 2012;14:804-818.
6. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. J Vet Intern Med 2012;26:1523.
7. Peterson ME. Nutritional management of endocrine disease in cats. Proceedings of the Royal Canin Feline Medicine Symposium 2013; 23-28 2013;23-28.
8. Teshima E, Brunetto MA, Vasconcellos RS, et al. Nutrient digestibility, but not mineral absorption, is age-dependent in cats. J Anim Physiol Anim Nutr (Berl) 2010;94:e251-258. 
9. Sparkes AH. Feeding old cats--an update on new nutritional therapies. Top Companion Anim Med 2011;26:37-42. 
10. Perez-Camargo G. Feline decline in key physiological reserves implications for mortality. Proceedings of the Nestlé Purina Companion Animal Nutrition Summit: Focus on Gerontology 2010;6-13. 
11. Freeman LM. Cachexia and sarcopenia: emerging syndromes of importance in dogs and cats. J Vet Intern Med 2012;26:3-17.

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