Hyperthyroidism is the most common glandular disorder of cats older than 8 years. It is usually caused by a benign tumor in one or both of the thyroid glands, which are located in the neck. These tumors cause the thyroid glands to overproduce thyroid hormones. The tumors are rarely malignant.

Thyroid hormones are important for regulating metabolism. When a cat’s thyroid hormone levels are too high, its metabolic rate increases, causing many body systems to work harder than normal. Problems associated with hyperthyroidism include damage to the heart, kidneys, liver, and eyes.

Cats with hyperthyroidism may show any or all of the following signs:

• Weight loss
• Increased appetite
• Vomiting
• Diarrhea
• Increased drinking and urination
• Matted hair/coat
• Hyperactivity

There are a number of options for treating hyperthyroidism in cats. The most common is administration of a drug called methimazole. This drug does not change the thyroid gland; instead, it interferes with the production of thyroid hormones. Therefore, it does not “cure” hyperthyroidism, and it must be given daily—usually twice a day—for the rest of the cat’s life. Often, it is given as a pill, but a gel that you can apply to your cat’s ear is also available. This treatment is relatively inexpensive and does not require surgery or a special facility. Most cats do very well with methimazole, but side effects may include vomiting and facial itching. Periodic blood testing is recommended for the duration of treatment, and dosage adjustments are sometimes required. Even if you and your veterinarian are planning to eventually treat your cat with radiation or surgery (see below), methimazole therapy is often used as a first step to resolve the signs of hyperthyroidism and make sure that they are not hiding, or masking, another condition (usually kidney disease).

A second and more permanent solution is treatment with radioactive iodine. This treatment destroys the thyroid tumors. While the procedure is relatively safe, the cat must remain at a special facility during treatment, which may require 7 to 10 days of hospitalization. Treatment with radioactive iodine usually cures the thyroid condition, so no pills are needed. Rarely, too much thyroid tissue is destroyed. Affected cats may need thyroid hormone supplements.

Another treatment option is surgical removal of the thyroid glands. If all of the abnormal tissue is removed, this treatment can cure hyperthyroidism. However, there may be anesthesia risks with older cats, and the surgery may result in complications.

Your veterinarian will discuss these different treatments with you further to help you decide which one is most appropriate for your cat.

One of the effects of hyperthyroidism is altered blood flow to the kidneys. In some cats with kidney disease, this effect makes it seem as if their kidneys are working normally. When methimazole is given to these cats and their metabolism slows down, the kidney disease can be diagnosed. Because kidney disease and hyperthyroidism have many signs in common, your veterinarian will ask to conduct tests of your cat’s blood and urine after starting methimazole therapy to check for kidney disease.

Most cats with uncomplicated hyperthyroidism—that is, they have hyperthyroidism and no other problems—do very well with treatment. In some cases, changes associated with high blood pressure, such as changes in the heart, improve or even completely resolve. If your cat has other conditions, such as kidney disease or high blood pressure that is not related to hyperthyroidism, you will need to treat these conditions as well. Your veterinarian will ask to test your cat for these conditions. Cats receiving methimazole therapy need to be regularly monitored with blood tests to make sure that the dosage is correct and that no other problems are developing. Left untreated, hyperthyroidism can lead to heart failure, sudden blindness from high blood pressure, chronic vomiting and diarrhea, and death. Hyperthyroidism is very common in older cats and very treatable, and most cats do well with drug therapy, radioactive therapy, or surgery.