Q & A: Persistent Hyperthyroidism in a Cat Treated with Hill's y/d

My patient is a 12-year old male spayed Siamese cat who has been hyperthyroid for 3 years. Over this time, his thyroid nodules have gradually increased in size and his methimazole dosage has increased from 2.5 mg once a day to 15.5 mg twice a day. Despite this large dose of methimazole, his serum T4 value remained in the high-normal to slightly hyperthyroid range.

After looking over the blood work and dosage and talking to the owner, we decided to try the new Hill's y/d diet. The cat has been on y/d food for over 3 months and her last serum T4 level was high-normal at 3.8 μg/dl. I usually prefer the values to be within the lower end of the reference range, so this seems to high for this cat.

Should I continue the y/d food and recheck or consider adding back the methimazole again? Why isn't this cat responding to either the methimazole or y/d diet?

My Response:

First of all, we must never forget that all cats with hyperthyroidism have a hyperfunctional thyroid tumor (1-6). These tumors are almost always benign at time of diagnosis, but they will usually continue to grow progressively larger when treated medically with either methimazole or carbimazole.

Although we don't know how the growth of these feline thyroid tumors will response to an iodine deficient diet, such as Hill's y/d, I would predict that the tumor will continue in its progressive growth pattern (1).

The fact that neither the methimazole or Hill's y/d is controlling the hyperthyroidism as well as you would expect is probably related to the fact that, after 3 years, the volume of this cat's thyroid tumor tissue is quite large. You could continue the y/d diet and reinstate a small dose of methimazole to the cat's treatment regimen, but you would need to monitor the cat's thyroid function very carefully over the next few weeks to ensure that hypothyroidism does not develop on the concomitant therapy.

Even if you can control the hyperthyroidism now, your cat could live a long time (she is a Siamese, after all). As she ages, the thyroid tumor will continue to grow and may transform into a thyroid carcinoma (7,8). In my studies, less than 2% of recently diagnosed hyperthyroid cats had thyroid carcinoma, whereas up to 20% of cats treated medically for longer than 4 years had evidence of thyroid carcinoma on thyroid imaging (7).

If this was my patient, I'd either do surgery or radioiodine to cure the cat's hyperthyroidism rather than trying to manage it with Hill's y/d or methimazole for the rest of her life. 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.

And again, she is a Siamese and most cats of that breed, when properly managed, seem to live almost forever!

References:

1. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; 571-592. 
3. 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.
4. 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.
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. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
7. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; in press.
8. 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.