Hyperthyroidism is the most common endocrine disease in middle-aged and senior cats. It is typically diagnosed when the cat is 12 to 13 years old.

Like the name implies, hyperthyroidism is the increased production and secretion of thyroid hormones from an abnormally functioning and enlarged thyroid gland. Because thyroid hormones affect almost every organ in a cat’s body, having excessive hormones typically causes secondary health issues, such as heart disease and high blood pressure. Hyperthyroidism can also mask kidney disease in cats.

In most cases, this disease is caused by a non-cancerous tumor, called an adenoma, on the thyroid gland. The exact cause of this disease in cats is not currently known.

Clinical Signs and Diagnosis

The most common sign of hyperthyroidism is weight loss in the face of an increased appetite. Over time, a cat with hyperthyroidism will become increasingly skinnier and may also lose muscle mass.

These cats may also experience increased thirst and urination, changes to their hair coat quality (more greasy, matted, or unkempt), vomiting, diarrhea, and increased vocalization.

Diagnosis of hyperthyroidism is through a complete physical examination as well as lab testing (complete blood count, chemistry, urinalysis, and thyroid hormones). If cat has hyperthyroidism, a thyroid hormone called T4 will be elevated on these lab tests.


There are currently four methods of treatment for hyperthyroidism; all have the goal of returning thyroid hormone levels to normal. Treatment options include medical therapy with antithyroid drugs; dietary therapy with iodine-restricted diets; surgical removal of the thyroid gland; and radioactive iodine therapy.

Medical Therapy

Methimazole is the most common anti-thyroid medication. It reduces the production and release of thyroid hormone from the thyroid gland. This therapy does not serve as a cure for the disease but allows for control.

Methimazole is relatively inexpensive and is either administered orally or as a transdermal gel that is applied to the ear. Both forms of medication are given twice a day. Some cats experience side effects of this medication within the first few months of starting it. These include vomiting, decreased appetite, lethargy, and skin itchiness that causes the cat to scratch and wound their face.

While on methimazole, repeated blood monitoring must be done to ensure the proper dosage is being used as well as to monitor kidney function and overall health. During the first 2 to 3 months of treatment, lab testing is done every 2 to 3 weeks. After thyroid hormone levels have normalized, monitoring is then done every 3 to 6 months.

Dietary Therapy

Feeding an iodine-restricted diet is another therapeutic option. This approach works because iodine is needed for the body to produce thyroid hormones. For the best results, this diet must be fed exclusively and consistently. These diets have been shown to reduce the levels of thyroid hormones in 3 to 4 weeks. If normal thyroid hormone levels are not achieved within 12 weeks, a different form of treatment is usually selected. Dietary therapy is not effective if the cat does not like to eat the food or regularly has access to other forms of food.  


Removal of the thyroid glands is another form of treatment. It is a straightforward procedure for a skilled surgeon and has a good success rate. Surgery has the advantage of being curative and eliminating the need for long-term medication administration. A risk of surgery is accidental damage to the parathyroid glands, which are located close to the thyroid glands. If the parathyroid glands are damaged, the cat’s body will have difficulty producing calcium. Because less invasive treatment options are available, surgery is rarely selected for treatment.

Radioactive Iodine Therapy

Radioactive iodine therapy is currently considered the treatment of choice for cats with hyperthyroidism. Radioactive iodine is given as an injection, which then travels through the bloodstream to the thyroid gland. The radiation from the radioactive iodine then destroys the abnormal thyroid tissue without damaging the surrounding healthy tissue.

The radioactivity does not harm the cat but does pose a risk for humans. Due to this, cats undergoing this treatment are hospitalized for 3 to 5 days after treatment until the radiation levels are low. Cats are not allowed visitors during this time. In 95% of cases, this treatment is curative within 3 months. If not successful, the treatment can be repeated. In rare cases, a cat can become hypothyroid (producing too low levels of thyroid hormone), which then must be treated with thyroid hormone supplement.


Once hyperthyroidism is diagnosed and a form of treatment is selected, most cats do well. As medical therapy and dietary therapy are not curative, treatment is lifelong in these cases. Surgery and radioactive iodine therapy are considered curative, so lifelong treatment is not needed.

If your cat is experiencing any of the clinical signs mentioned, please reach out to your veterinarian to have your cat tested. If your cat is diagnosed with hyperthyroidism, you and your veterinarian can determine the best treatment option for your cat.  

—Jeanette Barragan, DVM

Featured photo by Ekaterina Zagorska on Unsplash