Hyperthyroidism is a relatively commonly disease of middle-aged or older cats. It is usually caused by a benign tumor within the thyroid gland that secretes large amounts of thyroid hormone. One of the primary functions of this hormone is to regulate an animal's metabolism. Cats with too much circulating thyroid hormone have a greatly increased metabolic rate and frequently develop high blood pressure and a type of heart disease called hypertrophic cardiomyopathy.
Cats with hyperthyroidism tend to lose weight despite having a ravenous appetite. Vomiting, diarrhea, and increased thirst and urination are also possible. On physical exam, a veterinarian may hear a heart murmur, rapid heart rate, or abnormal heart rhythm or be able to feel an enlarged thyroid gland. Veterinarians will frequently order routine blood work, a urinalysis, and thyroid hormone levels to diagnose hyperthyroidism and to rule out other diseases that may cause similar symptoms (e.g., diabetes mellitus and kidney failure).
Several different types of treatment for hyperthyroidism are available. Many cats receive daily doses of an oral medication that suppresses the production of thyroid hormone. This therapeutic option is the least expensive to begin with, but treatment and monitoring need to continue for the rest of the cat's life. Injection of a mildly radioactive form of iodine or the surgical removal of the thyroid gland can permanently resolve many cases of hyperthyroidism. Although cats can be expected to live full and relatively normal lives with any of these three alternatives, radioactive iodine treatment is generally the most effective and carries the lowest risk of unwanted side effects.
Written by: Jennifer Coates, DVM
Last reviewed: October 2, 2008