The thyroid gland is located in the neck and secretes a hormone, called thyroid hormone, which circulates throughout the bloodstream and regulates the body’s metabolism. When thyroid hormone levels rise the body’s metabolism increases, causing an increase in such things as heart rate, activity level, appetite, and the rate at which calories are burned. When thyroid hormone levels decrease, the opposite is seen. In cats, the thyroid gland may develop a tumor, which, although rarely cancerous, begins to produce thyroid hormone at an excessive rate, resulting in a condition called “hyperthyroidism.”
Hyperthyroidism has been known to affect cats from ages 4 to 22 years, but the average age is about 12 to 13 years. Both sexes and all breeds of cats can develop the disease. The signs of hyperthyroidism are varied and include such things as weight loss, excessive hunger, a poor hair coat, excessive thirst, excessive urination, vomiting, hyperactivity, diarrhea, tremors, weakness, labored breathing, and in the advanced stages, loss of appetite and lethargy. If left untreated, hyperthyroidism can progress to cause such problems as heart disease, high blood pressure, extreme weight loss, dehydration, and a shortened life span. The diagnosis can be suspected on the basis of the history and physical examination (especially if a typical lump is found in the neck), but requires blood and urine tests to confirm as well as to rule out other conditions that may mimic hyperthyroidism. Occasionally, more in-depth tests, such as a nuclear thyroid scan, are required for a diagnosis.
Hyperthyroidism can be treated by one of four different methods. The best method is radioactive iodine therapy. This method is safe, painless, rarely has complications, and has a greater than 95% cure rate with just one treatment. The second method is by feeding an iodine-restricted prescription diet. This diet must be fed exclusively with no other foods or treats given. Dietary therapy does not cure the disease, but only controls it. Not all cats respond to this treatment method, but dietary therapy is safe and can be tried in just about any hyperthyroid cat. The third method of treatment is by using oral antithyroid drugs. Antithyroid drugs also do not cure the disease but only control it, must be given one to three times a day for life, and occasionally cause side effects which necessitates monitoring the patient with blood tests several times a year. Antithyroid drug treatment is, however, less expensive than radioactive iodine in the short term and many cats do well with this method of treatment. Antithyroid drug treatment is a viable alternative for cats in whom radiation treatment is not feasible and/or dietary therapy is not successful. The fourth method of treatment is surgical removal of the thyroid gland. Although surgery can produce a permanent cure, it is not done very often because of the increased risk with general anesthesia caused by hyperthyroidism and the comparable cost with a lower chance for a permanent cure when compared to radioactive iodine treatment.
Although hyperthyroidism can be a serious, life-threatening disease, it is often easily treated. With successful treatment, a hyperthyroid cat’s signs of disease can be reversed and it can go on to lead a normal life. If caught early before any clinical signs are evident, the prognosis is even better and the symptoms can be prevented.