Hyperthyroidism is the most common endocrine disease seen in cats.  Most often hyperthyroidism in cats is due to a thyroid enlargement called functional thyroid edematous hyperplasia.  Hyperthyroidism in dogs is much rarer.  In dogs, hyperthyroidism is most often caused by thyroid carcinoma.  Thyroid carcinoma is rare in cats.

The underlying cause of the thyroid enlargement (functional thyroid edematous hyperplasia) is not known but, may be due to the combined effects of several factors circulating in the blood.

Hyperthyroidism most commonly occurs in middle to old age cats. There is no sex or breed
predilection. In most cats hyperthyroidism is a slowly progressive disease which effects many organ systems. The most common clinical features of hyperthyroidism are weight loss, increased appetite, vomiting, increased water intake and urination and increased activity.  Less commonly decreased appetite, diarrhea, decreased activity, weakness and labored breathing are seen.

Diagnosis of hyperthyroidism is based upon clinical signs and simple blood tests.  Increased levels of thyroid hormones circulating in the blood are diagnostic. Normal levels of thyroid hormones in the blood do not necessarily rule out hyperthyroidism since these levels
are effected by other organ systems involved. A second blood test one to two weeks later is necessary in these cases.

Hyperthyroidism is treated medically with a drug called Tapazol which inhibits the production of thyroid hormone.  There are numerous side effects which may effect the treated cat including anorexia, vomiting and lethargy.  Surgical removal of one or both thyroid glands is an option which has fewer complications, although it is often necessary to normalize thyroid hormone levels medically prior to surgery.  Hypothyroidism is the most common complication of surgery.  This condition is easily treated with oral medication.