Hyperthyroidism occurs when the thyroid gland produces too much thyroid hormone. It occurs mostly in older cats (and is rare in dogs). The average age of cats with hyperthyroid disease is about 13 - only around 5% of cats with hyperthyroidism are younger than 10.

In hyperthyroidism, one or both of the thyroid glands (located in the neck) enlarge and overproduce thyroid hormone. It is most common for both glands to overproduce thyroid hormone than just one gland.

Because thyroid hormone affects the functioning of all of the organs, the signs for the disease can be quite variable. Some of the most common signs will include increased appetite with weight loss, increased restlessness, aggressive behavior, an elevated heart rate, increased water consumption and increased urination, vomiting, increased stool size, diarrhea, dull haircoat, difficulty breathing, and depression and weakness.

The first step is correct diagnosis. The first examination will be to palpate the thyroid glands in the neck and see if they are enlarged. Diagnosis of this disease is made when the level of thyroid hormone is found in increased levels in the blood. Most hyperthyroid cats have very high levels of hormone, so it is usually very straightforward. But some cats will have signs of hyperthyroidism with normal or only slightly increased levels of thyroid hormone. And, thyroid hormone levels may vary over time, so it might be necessary to check blood levels several times.

One conventional treatment is with Radioactive Iodine Therapy. This may sound odd, but this is a very effective way to treat hyperthyroidism. It is given by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires one or two weeks of hospitalization at a veterinary hospital licensed to administer radiation therapy. This treatment is often limited to veterinary teaching institutions because of governmental regulations regarding radioactive materials. Sometimes, but not always, the expense can be greater than for the other options.

A second traditional treatment is with Oral Medication. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Some cats have reactions to the drug (approximately 20%). However, the side-effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Methimazole does not destroy the abnormal thyroid tissue but rather prevents the production of excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated. This type of treatment is appropriate for the cat who is a poor surgical risk due to other health problems or is exceptionally old. It may also be used for a few weeks to stabilize the cat who is at increased surgical risk because of cardiac complications.

A thyroidectomy, which is surgery to remove the abnormal tissue, is another alternative. But with surgery, recurrence of hyperthyroidism is a possibility with some cats. If surgery is done as a treatment, it is still possible to see recurrence if abnormal thyroid cells remain. Any remaining cells may grow, and this will cause recurrence of the disease. Overall, recurrence is less than 5%, and usually later than 2-4 years after treatment. An additional possibility for recurrence is that at the time of surgery, one lobe of the thyroid gland was normal, so it was not removed. Then years later it became abnormal, and the disease reoccurs.

One alternative treatment would be with Bugleweed (Lycopus Europeus) and Melissa (Melissa Officinalis). These are two herbs that have been used in combination to combat the effects of hyperthyroid disease. Bugleweed has been shown to decrease thyroid hormone levels in rats 24 hours after administration. The standard dose is 1 drop per lb of body weight of the tinctures given twice daily.

Carnitine, an amino acid health supplement, is another alternative treatment. Carnitine, an ammonium compound, is normally synthesized within the body from the amino acids lysine or methionine (primarily in the liver and kidneys). If administered as a food supplement, Carnitine seems to be effective in reversing the signs of hyperthyroid disease in humans. A starting dose for a cat is 250 mg per day.

Hyperthyroid disease in cats is a very serious disease. It is a very common endocrine disorder. About 2% of all cats greater than the age of 10 will contract hyperthyroidism. However, the disease is relatively easy to diagnose and correctly administered treatment has a quite high success rate. Cats who have been treated for the disease are able to lead healthy, normal lives.