Cat Hyperthyroidism: Facts and Statistics

Hyperthyroidism is a disorder of the thyroid gland and it is the most common glandular disorder affecting cats. The thyroid gland is responsible for producing and secreting thyroid hormone, also known as thyroxine. Thyroxine is responsible for regulating your cat’s metabolism. If your cat has hyperthyroidism, it means that too much thyroxine is produced, causing your cat’s metabolism to increase, which stresses your cat’s organs, including heart, kidneys, nervous system, gastrointestinal tract, and liver.

Hyperthyroidism in Cats

Your cat has two thyroid glands located in the neck. Hyperthyroidism can occur in any breed of cat, male or female, but is most common in middle-aged to older animals. According to the ASPCA (//, less than 6 percent of cases are younger than 10 years of age and the average age at onset is between 12 and 13 years. Fewer than 2% of hyperthyroid cases in cats involve malignant thyroid gland tumors.

Besides age, there are environmental risks that may predispose some cats to hyperthyroidism, including exposure to high levels of dietary iodine by cats that are already likely to develop hyperthyroidism. Although no specific breed has been clinically proven to have a greater risk of hyperthyroidism, there is a somewhat increased incidence of hyperthyroidism in Siamese cats.

Clinical Signs of Hyperthyroidism

The average age of cats with hyperthyroidism is approximately 12 years and about 5% of hyperthyroid cats are younger than 10 years of age. The most common clinical sign of hyperthyroidism is weight loss, as well as excessive thirst, increased urination, hyperactivity, unkempt appearance, shedding, periodic vomiting, panting, diarrhea, aggression and increased vocalization, especially at night.

Secondary complications include hypertension or high blood pressure and thyrotoxic cardiomyopathy. In some instances, high blood pressure can cause retinal bleeding or retinal detachment, resulting in blindness.

Diagnosis of Hyperthyroidism in Cats

Because cats with suspected hyperthyroidism are often geriatric and concurrent illness such as diabetes, inflammatory bowel disease, intestinal cancer or chronic kidney failure can have overlapping symptoms, diagnosis of hyperthyroidism in these cats can be complicated. The first step is to determine the level of thyroid hormone or T4 in your cat’s blood. However, if your cat has a T4 level that is in the upper range of normal but hyperthyroidism is still suspected a second test is performed. If these tests are not definitive, your cat’s T4 can be measured again in a few weeks.

To find out more about the diagnosis of cats with hyperthyroidism, please read this study (// published in the Journal of Veterinary Internal Medicine (//

Treating Your Cat’s Hyperthyroidism with Radioactive Iodine

Radioactive iodine therapy is a very effective way to treat hyperthyroidism. Radioactive iodine destroys the abnormal thyroid tissue without endangering your cat’s other organs. The treatment does not require anesthesia. Recurrence of hyperthyroidism is rare after radioactive iodine therapy. You can read more about treatment options for your cat in a study published by The Canadian Veterinary Journal (// and available on the National Institutes of Health website.

**Back to Feline Hyperthyroidism: An Overview

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