Hyperthyroidism in Cats: Learn the Signs

Hyperthyroidism in cats

If you’ve noticed your senior cat has lost some weight without a change in its diet, or your furry friend is being a lot more active than usual, it could be your cat is suffering from hyperthyroidism. If left untreated, hyperthyroidism can lead to heart failure.

What is Hyperthyroidism in Cats?

More common in cats than dogs, hyperthyroidism occurs when a cat’s thyroid glands produce an excess an amount of the thyroid hormone.

Are senior cats more prone to hyperthyroidism?

Yes. According to an article from the Washington State University’s College of Veterinary Medicine, “The average age of cats with hyperthyroidism is 13 years of age; only about 5% of hyperthyroid cates are younger than 10 years of age.”* Hyperthyroidism in cats is not breed or gender specific.

How can I tell if my cat is suffering from hyperthyroidism?

Since the thyroid hormone can affect various organs in a cat, the signs of hyperthyroidism can vary greatly. Common signs your cat might have hyperthyroidism include: weight loss, increase in activity and appetite, an unusually higher heart rate, a seemingly unquenchable thirst, vomiting, diarrhea, increased urination, or difficulty breathing. If your cat is experiencing these signs, take it to a veterinarian as soon as possible for a proper diagnosis.

How is hyperthyroidism in cats diagnosed?

A cat with hyperthyroidism will have one or both of its thyroid glands enlarged. Abnormal thyroid hormone levels can be detected in the blood, so your veterinarian will conduct a complete blood work to confirm if your cat has hyperthyroidism and to rule out any other disease.  In cases when hyperthyroidism cannot be detected by blood work, your veterinarian will want to perform a nuclear medicine scan of the thyroid. For this test, your cat will be administered a small dose of a radioactive compound into its bloodstream. If your cat’s thyroid glands are hyperactive, it will absorb most of the compound. A short hospital stay will be required if this test is performed.

How is hyperthyroidism in cats treated?

The three most common types of treatment are radioactive iodine therapy, removing the thyroid gland surgically, and administering anti-thyroid medication orally. Consultation with a veterinarian is strongly recommended before determining a treatment method.

Radioactive Iodine Therapy – Treatment consists of injecting the cat with radioactive iodine. The iodine works its way to the afflicted thyroid gland where it destroys the abnormal thyroid tissue without harming the normal tissue. This treatment is the most effective, but it is also the most costly, and it requires a one- to three-week stay in an animal hospital that specializes in this treatment.

Surgical Removal of the Thyroid Gland – This method requires the removal of the thyroid glands. Costly, but effective, this procedure will help reduce the need for long-term thyroid medications.

Anti-Thyroid Medication – This method simply requires the oral administration of an anti-thyroid medication to your cat. Less invasive than the previous two treatments, having to administer your cat anti-thyroid medication is a lifetime commitment.

Methimazole Hyperthyroidism in Cats

Methimazole Tablets

Methimazole  – This medication has been proven highly effective in the treatment of hyperthyroidism. Once treatment begins, methimazole is known to provide rapid positive results. It is also used before thyroid surgery or radioactive treatment. In addition, methimazole can help prevent enlargement of the heart and damage to other vital organs.

felimazole hyperthyroidism in cats

Felimazole Tablets

Felimazole is approved for the treatment of feline hyperthyroidism. It is the first FDA approved treatment for feline hyperthyroidism. Felimazole contains the anti-thyroid drug methimazole, which reduces the production of T4 and T3 within the thyroid gland.



In summary, senior cats are prone to hyperthyroidism which occurs when one or both of the thyroid glands over produce the thyroid hormone. Left untreated, hyperthyroidism can lead to heart failure. Bloodwork and/or a nuclear medicine scan are the two most common ways to diagnose hyperthyroidism in cats. There are three ways to treat hyperthyroidism: radioactive iodine therapy, surgically removing the affected gland(s), and administering ant-thyroid medication. Each treatment has is benefits and risks. Therefore, it is important to discuss the various treatment methods with a veterinarian. With the proper treatment, a cat with hyperthyroidism should be able to lead a normal life.

Does your cat have hyperthyroidism? If yes, please share with us in the comments which treatment method you chose.


*Source: Washington State University College of Veterinary Medicine. “Hyperthyroidism in Cats”. https://www.vetmed.wsu.edu/outreach/Pet-Health-Topics/categories/diseases/hyperthyroidism-in-cats Accessed on 12/27/16.

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