I have a 7-year old female spayed Siamese cat that has been on Hill's y/d food for the past 2 months. The cat's thyroid levels have not come down (still > 15 μg/dl) and she is miserable on the food. She acts like she's starving (possibly from the uncontrolled thyroid) and is trying to steal food from the owner.

I am discontinuing the food but was wondering if I need to wait a certain amount of time before starting her back on methimazole?

My Response:

I've heard the same story from many cat owners and veterinarians. The y/d just doesn't appear to look and taste very appetizing to many cats (the food certainly looks pretty gross to me, but I must admit — I haven't tasted it!).

If the cat's serum T4 value is still high now, I'd switch the cat to a good diet (higher in protein, lower in carbs) that she wants to eat and start methimazole now.

• A 7-year old Siamese could live a very long time, possibly another 15 years! I'd talk to the owners about the following facts when you discuss how to treat the cat's hyperthyroidism (1-5): 
• This cat, like all hyperthyroid cats, has a thyroid tumor
• The thyroid tumor and hyperthyroidism will never go into spontaneous remission
• The thyroid tumor will continue to grow larger with time
• In some cats treated long term medically, the benign tumor will transform into a malignant thyroid carcinoma. In my studies, the incidence of thyroid carcinoma is above 20% in cats treated medically for longer than 4 years (6).

The bottom line:
If this was my own cat, I'd either do surgery or radioiodine to cure the cat's hyperthyroidism rather than trying to manage it with an iodine deficient diet (Hill's y/d) or methimazole for the rest of her life (1-5,7,8). She is already suffering from a severe case of hyperthyroidism, which will only worsen with time.

In the long run, a definitive treatment would be the wisest (and most cost effective) means of treating this relatively young cat.

References & Suggested Reading:

1. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; 571-592. 
2. Peterson ME: Hyperthyroidism, In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Fifth Edition). Philadelphia, WB Saunders Co. 2000; pp 1400-1419.
3. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008, pp 127-168.
4. 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. 
5. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
6. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthryoidism. Journal of Veterinary Internal Medicine 2012; in press.
7. Peterson ME: Radioiodine treatment for hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:34-39.
8. Peterson ME: Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC (eds): Current Veterinary Therapy XIV. Philadelphia, Saunders Elsevier, 2008, pp 180-184.