• Decrease font size
  • Return font size to normal
  • Increase font size
U.S. Department of Health and Human Services

Animal & Veterinary

  • Print
  • Share
  • E-mail

FDA Approves First Drug to Treat Hyperthyroidism in Cats

by Melanie McLean, D.V.M., Communications Staff
FDA Veterinarian Newsletter 2008 Volume XXIII, No. VI

“My 12-year-old cat ‘Bear’ has been having diarrhea off and on for awhile now. He’s always hungry, but he’s getting skinnier. He never seems full even though I’m feeding him five meals a day. I see him at his water bowl a lot, too.” A small animal veterinarian hears variations of Bear’s owner’s story often, and feline hyperthyroidism is always on the top of the rule-out list.

In the past, when hyperthyroidism was the diagnosis, there was no drug approved specifically for cats to treat this disease. Now that has changed.

In May 2009, the Food and Drug Administration approved FELIMAZOLE (methimazole). FELIMAZOLE, manufactured by Dechra, Ltd., in Staffordshire, United Kingdom, is the first, and currently only, FDA-approved drug for the treatment of hyperthyroidism in cats.

Also called thyrotoxicosis, hyperthyroidism is an endocrine disease that results from the over-production of thyroid hormones by the thyroid gland. FELIMAZOLE works by blocking this over-production.

Hyperthyroidism is the most common endocrine disease of cats older than 8 years of age. In almost 99 percent of the cases, it is caused by a benign tumor of the thyroid gland. The butterfly-shaped thyroid gland is located in the neck, with one lobe on each side. It plays an important role in regulating the body’s “engine,” or metabolic rate. When the thyroid gland produces an excessive amount of thyroid hormones, it causes the cat’s “engine” to run at an abnormally high speed. Almost all of the cat’s organs are affected by this high metabolic rate.

The most common clinical sign of hyperthyroidism in cats is weight loss despite an increased appetite. Other common clinical signs include vomiting, diarrhea, hyperactivity, drinking and urinating more than normal, and an unkempt hair coat. Because the disease develops gradually, many cat owners miss the early signs of hyperthyroidism, causing a delay in diagnosis and treatment.

Hyperthyroidism often leads to high blood pressure (hypertension) and heart disease. Hypertension is a consequence of the increased pumping pressure of the heart. In some cats, the blood pressure becomes so high that retinal hemorrhage or detachment occurs, resulting in sudden blindness. Heart disease develops because the heart must pump faster and more forcefully to meet the body’s increased metabolic demands. To compensate for this increased workload, the muscles of the heart thicken, causing heart enlargement and eventual heart failure. The mortality rate of untreated hyperthyroidism is almost 100 percent.

A veterinarian may suspect that a cat has hyperthyroidism by the clinical signs described by its owner and by feeling the enlarged thyroid gland in its neck. The most common way to confirm the diagnosis is a blood test that measures the level of one of the thyroid hormones called thyroxine (T4). This is referred to as the cat’s total T4 (TT4) concentration.

One treatment option for hyperthyroidism is oral medication, which can be given life-long or to stabilize the cat prior to other treatment options, such as radioactive iodine therapy or surgery. Until FELIMAZOLE, there was no approved oral medication available for veterinary use in the United States. For years, veterinarians have used the human-approved form of methimazole in an extralabel (“off label”) manner in cats. But now there is FELIMAZOLE, a veterinary-approved form of methimazole. Unlike the human methimazole products, the effectiveness and safety of FELIMAZOLE have been evaluated specifically in cats and the product label provides dosing and safety information specific to the cat.