Feline Hyperthyroidism
Hyperthyroidism is a common disease that occurs in older cats as a result of excessive production of thyroid hormone. A benign growth or adenoma of the thyroid gland in the neck is responsible in most cases for excessive production of the hormone. This speeds up the body's metabolism and affected cats tend to lose weight despite ravenous appetites. Other signs can include excessive thirst and urination, frequent vomiting, diarrhea or constipation, elevated heart rate, hyper-excitability and nervousness. These signs are often gradual in onset and may not be obvious to most owners for 6-12 months.
If hyperthyroidism is suspected, bloodwork is performed. Baseline thyroid levels are often markedly elevated relative to those of normal cats. Occasionally some affected cats have other diseases. For example, many geriatric cats suffer from inadequate kidney function and heart disease. Further diagnostics and additional treatments may be required to deal with these other problems.
Treatment for cats with hyperthyroidism usually includes the administration of the anti-thyroid drug methimazole (Tapazole). Radioactive iodine or surgical removal of both thyroid glands are alternative choices. There are advantages and disadvantages to each method and the choice for each individual patient should be discussed with your veterinarian.
Methimazole (Tapazole) is relatively inexpensive, readily available and most owners can administer it at home. The medication is usually given once to twice daily and is necessary for the rest of the cat's life. Re-evaluation of the thyroid levels is necessary after initiating treatment. Mild side effects of lethargy, inappetence and infrequent vomiting often resolve after the first two weeks of Tapazole use. Severe side effects are less common and can include liver complications, blood abnormalities and an itchy face which are readily reversible when the medication is stopped. Hyperthyroidism is a progressive disease and there is often need to adjust the dosage periodically. The prognosis for most cats with hyperthyroidism is good to excellent. It is important to remember that treatment with methimazole (Tapazole) results in control of clinical signs. It does not result in a cure.
Radioactive iodine is the preferred treatment in that it is curative in approximately 80 percent of cases. It has few side effects and treats bilateral disease and any ectopic tissue (thyroid tissue outside the thyroid glands).