What Is it?

Hyperthyroidism is a multisystemic disorder in cats resulting from the excessive production and secretion of thyroid hormones by the thyroid gland. This is almost always the result of an intrinsic disorder of the thyroid gland itself, either hyperplasia (increased number of thyroid hormone-producing cells), or less commonly, benign tumors (adenomas) or malignant tumors (carcinomas) of the thyroid gland. Approximately 20% of cats will only have 1 lobe affected, whereas the other 70-80% have bilateral involvement. Occasionally, animals will have ectopic thyroid tissue in the lower neck or chest that is causing the signs of hyperthyroidism. We have yet to uncover what stimulates this excessive growth or tumor formation within the thyroid gland, but both environment and diet have been implicated.

Hyperthyroidism is most common in cats older than 8 years of age and there is no sex predilection. Because thyroid hormones are responsible for the body’s metabolism as well as several bodily functions, an overproduction of these hormones can lead to a variety of clinical signs. These may include weight loss, increased appetite, hyperactivity, restlessness, haircoat changes, increased water consumption and urination, vomiting, diarrhea, and aggressive behavior. Occasionally, cats will present with uncommon signs such as lethargy, weakness, and a lack of appetite. The owner may also notice an enlargement in the neck. Because this disease causes such a variety of signs, it should always be suspected in any older cat with medical problems.

Cats with hyperthyroidism may have concurrent problems with their heart and kidneys. Hypertrophic cardiomyopathy is a condition of the heart in which the walls of the heart thicken and the heart has difficulty adequately pumping blood to the body. Your veterinarian may detect an increased heart rate, heart murmurs, rhythm disturbances, or signs of heart failure on physical examination. Very rarely, these cats will present with the dilative form of cardiomyopathy in which the heart walls have become thinner and the heart larger, resulting in a decreased ability to pump blood. In addition, many cats with hyperthyroidism have concurrent renal failure (approximately 30%). Often times, there may not be clinical signs of renal failure because the hyperthyroid cat has increased blood flow and perfusion to the kidneys. In these cases, treatment of the hyperthyroid state may lead to a decompensation of the kidney failure due to a decrease in blood flow.


Diagnosis of hyperthyroidism begins with a complete medical history and a thorough physical examination. A minimum data base including a complete blood count (CBC), a serum chemistry profile, and a urinalysis can help detect concurrent diseases and establish baseline data prior to treatment.

Hyperthyroidism is often diagnosed on the basis of clinical signs, a palpable thyroid nodule in the neck, and the finding of an increased serum T4 concentration. Thyroxine or T4 is one of the thyroid hormones produced by the thyroid gland and is elevated in the majority of hyperthyroid cats. However, it is possible for this test to be in the normal range in cats with mild hyperthyroidism or in cats that have a concurrent illness lowering the T4 levels. If your veterinarian strongly suspects hyperthyroidism and this test initially is normal, he or she may suggest repeating the test or submitting a serum free T4 test (a more specific version of the hormone). Other diagnostics that can be performed include a T3 suppression test or a TRH stimulation test, both tests that assess the thyroid hormones and their pathways in the body. These, however, are rarely performed. Occasionally radionuclide thyroid scanning is necessary. With this test, a radioactive substance is injected into the bloodstream. It is preferentially taken up by functioning thyroid cells and can be very useful in detecting ectopic thyroid tissue.

If concurrent diseases are suspected, chest x-rays, abdominal x-rays, and an echocardiogram (ultrasound) of the heart may be warranted.


There are three modes of therapy for hyperthyroidism: thyroidectomy (surgical removal of the thyroid gland), oral antithyroid medications, and radioactive iodine. Both thyroidectomy and radioactive iodine are aimed at curing the disease, while oral antithyroid medications serve only to control the signs of hyperthyroidism. The mode of therapy depends upon the age and health of the cat, the status of the renal function, concurrent diseases, unilateral vs. bilateral involvement, and the owner’s wishes.

Antithyroid medications inhibit the synthesis of thyroid hormones. These medications are indicated for a treatment trial to assess the effect of resolving hyperthyroidism on renal function, alleviate symptoms prior to surgery or radioactive iodine therapy, or as long term therapy. Methimazole (Tapazole) is the medication of choice. This medication is started at a low dose range for 2-4 weeks, then the T4, CBC, and profile are rechecked and the dose is altered accordingly. Ideally, the CBC, platelet count, and chemistry panel are performed frequently during the first 3 months of treatment. After that, testing can be done every 3-6 months. Adverse effects of methimazole include a decreased appetite, vomiting, lethargy, liver disease, bleeding episodes, decreased numbers of platelets, and other blood cell abnormalities. The major disadvantage of this treatment, aside from the risks of adverse side effects, is that it must be given once to twice a day for the life of the cat.

Radioactive iodine therapy is an excellent mode of therapy that can result in a clinical cure of hyperthyroidism. Iodine 131 (I131) is concentrated within the thyroid gland and the emitted radiation destroys the surrounding functioning thyroid cells without causing radiation damage to surrounding structures. Any cells that are atrophied and nonfunctioning are spared (and later become functional once the hyperthyroidism is controlled), which is why most cats treated this way do not have problems with hypothyroidism later in life (decreased thyroid hormones). Approximately 2% of cats will become hypothyroid, 2-4% will require a second I131 treatment, and 2% will have a recurrence of the disease within 1-6 years of treatment. The disadvantage of this treatment is a prolonged hospitalization while the cat is considered to be ‘radioactive’.

Finally, surgical thyroidectomy can be performed to remove the diseased thyroid gland(s). This ideally offers a complete cure, though clinical signs of hyperthyroidism may recur months to years after surgery. Clinical signs will also persist if there is any ectopic thyroid tissue, if a small portion of the thyroid was left in place, or if a malignant thyroid carcinoma had metastasized to other locations. The biggest risk of surgical thyroidectomy is the removal of the parathyroid glands. These are small glands located in close proximity to the thyroid glands. They are necessary for calcium balance in the body and their removal results in the serious complication of hypocalcemia (low calcium). In addition, thyroidectomy is contraindicated if the risk of anesthesia is too great or if renal function is poor.

It is best to discuss these possible modes of treatment thoroughly with your veterinarian and determine which one is best for your cat. Prognosis varies depending on the individual case and can be very difficult to predict due to the possibility of secondary complications of both the disease and of the treatments.

Content prepared by St. Francis Animal Hospital, 1227 Larpenteur Ave. West, Roseville MN. 55113