FDA OKs First Hyperthyroidism Drug for Cats

Drug, Called Felimazole, Treats Overactive Thyroid in Cats

Reviewed by Audrey Cook, BVM&S on July 20, 2009

June 9, 2009 -- The FDA has approved Felimazole, the first drug to treat hyperthyroidism in cats.

Hyperthyroidism is caused by an increase in production of thyroid hormones from one or both of the thyroid glands.

Felimazole's active ingredient, methimazole, is an antithyroid drug. It works by blocking the creation of thyroid hormones. Although human formulations of this drug have been used for many years to treat cats with hyperthyroidism, Felimazole is the first veterinary-approved formulation. It is manufactured in a smaller tablet size to facilitate accurate dosing in cats.

"Hyperthyroidism is one of the most common diseases seen in middle-aged and senior cats," Bernadette Dunham, DVM, PhD, director of FDA's Center of Veterinary Medicine, states in an FDA news release.

"Cats can become seriously ill with this condition. The approval of Felimazole offers cat owners, in consultation with their veterinarians, an effective medical treatment for this serious disease," Dunham says.

Thyroid hormones play an important role in controlling the body's metabolic rate. Hyperthyroid cats generally have weight loss, despite increased appetite and food intake. The disease can also cause increased thirst, hyperactivity, rapid heart rate, and irritability. Left untreated, the disease can result in heart failure or high blood pressure.

The safety and effectiveness of Felimazole were evaluated in studies done in the U.K. and U.S.

In those studies, most cats with hyperthyroidism were successfully treated with Felimazole. The most common side effects associated with Felimazole included a change in food consumption (increase or decrease), lethargy, vomiting, and diarrhea. Less common side effects included anemia and low platelet counts in some cats. These issues are easily identified with routine laboratory tests.

Felimazole is made by Dechra, Ltd. in Staffordshire, England.