Hyperthyroidism is a condition in which there are higher than normal levels of thyroid hormones in the blood because the thyroid gland produces excessive amounts. Cats are more prone to hyperthyroidism than dogs. A thyroid tumor is the most common cause of hyperthyroidism in cats. Treatment may involve surgery, irradiation, and drugs.

Thyroid hormones regulate the body's overall production of energy and metabolism. Common signs of hyperthyroidism include weight loss, hyperactivity, increased appetite, increased heart rate, increased water intake and urination, increased stool volume, vomiting and diarrhea. Also, the thyroid gland will be enlarged. Bloodwork will show elevated levels of thyroid hormones to confirm the diagnosis.

Upon diagnosis, your veterinarian will refer you to a veterinary oncologist that specializes in hyperthyroid cats. Hyperthyroidism is generally best treated by thyroidectomy, surgical removal of the thyroid gland, and it may or may not be followed by thyroxine supplementation. However, if your cat is not a good surgical candidate, you may elect to treat with radioactive iodine or antithyroid drugs. Radioiodine treatment requires no anesthesia and involves injecting radioactive iodine intravenously into the body that destroys the thyroid tumor. Treatment with an antithyroid drug, methimazole (Tapazole), controls hyperthyroidism by blocking the production of thyroid hormones. This is a lifelong medication that must be given daily. Side effects of methimazole include vomiting, loss of appetite, liver problems, bleeding and changes in white blood cell counts. Therefore, it is important to monitor for adverse reactions during the first 3 months of treatment with several intervals of complete blood counts and thyroid tests. Response to treatment will continue to be monitored at 3 to 6 month intervals thereafter with thyroid tests and methimazole dosages to be altered as needed.

1.The Merck Veterinary Manual, Merck & Co., Inc., Rahway, N.J., p. 284-285, 1991
2. Bill, Robert (Pete), Pharmacology for Veterinary Technicians, American Veterinary Publications, Inc., Goleta, CA, p.222-227, 1993