FDA Approves First Drug to Treat Feline Hyperthyroidism

June 9, 2009

The Food and Drug Administration announced today the approval of Felimazole (methimazole), a new drug for the treatment of hyperthyroidism in cats. Felimazole, a new molecular entity for animals, is the first drug approved for hyperthyroidism in cats.

Hyperthyroidism is caused by an increase in production of thyroid hormones from the thyroid glands. Methimazole is an antithyroid drug and works by blocking the creation of thyroid hormones.

"Hyperthyroidism is one of the most common diseases seen in middle-aged and senior cats," said Bernadette Dunham, D.V.M., Ph.D., director of FDA's Center for Veterinary Medicine. "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.”

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 effectiveness and safety of Felimazole were evaluated in field studies conducted in the United Kingdom and the United States. Success was measured by a decrease in the production of thyroid hormones or T4 levels and improvement in clinical signs, i.e., weight gain, no longer vomiting, return to normal eating habits, etc.  A total of 166 hyperthyroid cats were treated with Felimazole.  By day 42, 77% of cats in the UK study had normal T4 levels and 81% had reduced disease severity. In the US study, by day 42, 61% of cats were considered treatment successes with a significant decrease in T4 concentration and improved clinical signs.

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 include anemia and low platelet count in some cats.

Felimazole is manufactured by Dechra, Ltd., Staffordshire, United Kingdom.

For more information, please see the FOI summary.