The thyroid gland lies along the sides of the trachea at the base of the neck, and is essential for life and influences nearly every cell in the body. In the disease known as hyperthyroidism, the thyroid gland enlarges as functioning thyroid tissue becomes hyperplastic. This proliferation of thyroid tissue results in the production of excessive amounts of the hormones triiodothyronine (T3) and thyroxine (T4). Hyperthyroidism is seen in middle-aged to older cats, at an average age of 13.

Cats with this disease are like shooting stars-blazing in activity until they burn themselves out. Enlarged hearts are common in cats with hyperthyroidism. Other symptoms include increased thirst and urination, periodic vomiting and diarrhea, unkempt coat and hyperactivity. Laboratory tests are needed to confirm a diagnosis, but basic procedures to aid in diagnosing feline hyperthyroidism include palpation (if the thyroid gland can be palpated, it is abnormal) and taking note of a rapid heartbeat.

There are currently three ways to treat this disease: antithyroid drugs, surgery to remove adenomatous thyroid tissue and administration of radioactive iodine. Regardless of the method used, the goal is to bring circulating thyroid hormones into normal range. Research indicates that hyperthyroid cats live an average of two years with treatment.