Hyperthyroidism in Dogs ~ Pawprints and Purrs, Inc.

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Hyperthyroidism in Dogs

Hyperthyroidism is an endocrine disease seen in cats and dogs. Most often hyperthyroidism is due to a thyroid enlargement called functional thyroid edematous hyperplasia. Hyperthyroidism is rare in dogs. Canine hyperthyroidism is most often caused by thyroid carcinoma. Thyroid carcinoma is rare in felines.

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, and more recently, dogs. There is no sex or breed predilection, though some dog breeds have had more case files than others. Hyperthyroidism is a slowly progressive disease which effects many organ systems. The most common clinical features of hyperthyroidism are elevated heart rate, weight loss, increased appetite, nervousness, increased water intake and urination, and increased activity. Less commonly seen are decreased appetite, decreased activity, weakness and labored breathing.

Diagnosis of hyperthyroidism is based upon clinical signs and simple blood tests called T4. This is a simple and innexpensive test, but it can be falsely low due to drugs or non-thyroid illness for both hyperthyroidism and hypothyroidism. A test called Free T4 (or FT4) is much better because it reflects true thyroid function. Free T4 testing is relatively expensive.

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 animal 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, Thyroxine.

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Resource Reference:
The Dog Owner's Health Manual, Stillman, John D., DVM