Older cats can lose weight for a number of reasons. One frequent cause of weight loss in cats is hyperthyroidism. In this disease, one or both lobes of the thyroid gland become enlarged and secrete excessive amounts of the thyroid hormone.

The abnormally high levels of the thyroid hormone cause the cat’s metabolism to be in a hyperactive state. This results in several detrimental changes to the cat’s system. Fat and muscle are consumed for energy, resulting in weight loss. The body’s accelerated demand for oxygen increase the workload on the heart, causing heart disease and high blood pressure. Kidney and liver function can also be adversely affected by the changes in metabolism.

Hyperthyroid cats can show many different symptoms in addition to weight loss such as increased or decreased appetite, fever, increased water consumption, hyperactivity or weakness, vomiting or diarrhea, unkempt hair coat, and rapid nail growth.

Fortunately, the disease can be successfully treated, especially if it is detected in its early stages. Left untreated, total body starvation and heart failure result. Therapy for hyperthyroidism involves either food, medication, surgery, or radioactive iodine. Initially, most cats are given anti-thyroid medication, methimazole, to restore the thyroid levels to normal. However, although medical therapy will control the disease, it will not cure it. Surgical removal of the thyroid gland or administration of radioactive iodine are curative. The various methods of treatment are discussed in more detail below.

Food: Hill's Prescription Diets recently developed a low iodine food that has been found to be very effective in treating hyperthyroidism. It comes packaged as a dry formula and canned. If you choose to try this option, you can only feed your cat this diet. Treats must be made out of the diet. You must also start with new dishes for feeding.

This option works best in single cat households, or where it is possible to feed the cats separately. If the cat won't eat the diet, then you may choose one of the other options.

Medication: If medical therapy is chosen for long term treatment, the medication will need to be given for the rest of your cat’s life. The dose may need to be adjusted periodically as fluctuations in the levels of thyroid hormone occur. Although the drug is generally safe, side effects can occur including vomiting, loss of appetite, bleeding abnormalities, changes in blood cell counts, liver toxicity, or allergic reactions. Serious problems can be detected on routine blood work, and so while your cat is taking methimazole, we will perform blood tests every 3-4 months.

Surgery: Surgical removal of one or both lobes of the thyroid gland will remove the tissue that is secreting abnormal amounts of the thyroid hormone. Once removed, the thyroid gland is examined microscopically by a pathologist to be sure that the tissue is not cancerous. (This occurs in only a small number of cases.) The parathyroid gland is a small gland that lies in very close proximity to the thyroid gland; it controls calcium metabolism. Although it is not removed during surgery, it can sometimes temporarily shut down. For this reason, cats that have both lobes of the thyroid gland removed will have their blood calcium levels monitored for several days after surgery. Surgery is an effective means of treating hyperthyroidism. Complications are rare but can occur, and include accessory thyroid tissue or a need for thyroid or calcium supplementation.

Radioactive iodine: Radioactive iodine is injected subcutaneously. It is taken up by the thyroid gland where it selectively destroys thyroid tissue while leaving other tissues unharmed. This treatment requires special facilities, and is performed on a referral basis in St. Louis. Hospitalization is required until most of the radioactive iodine has been excreted from the cat’s body, typically about 3-4 days. Treatment with radioactive iodine is an effective means of treating hyperthyroidism. Complications are rare but can occur, and include incomplete destruction of thyroid tissue or a need for thyroid supplementation.