Cat Hyperthyroidism

Is your older cat losing weight yet eating plenty, perhaps even more than he/she used to? How about vomiting, restlessness, increased thirst, vocalization? If some or all of these symptoms are present, then your cat may have hyperthyroidism. Hyperthyroidism results from excess levels of thyroid hormone being released from benign nodules in the thyroid gland. A normal thyroid gland receives “feedback” from the body to control the appropriate level of hormone. In the hyperthyroid state, however, these nodules ignore such feedback and continue to release thyroid hormone into the bloodstream in excess of that required.

The effect on the cat is often profound, as this hormone controls the rate of metabolism. With increasing levels, many body systems are affected; heart rate rises, energy consumption increases so food intake often escalates to try and keep pace with metabolic needs. Despite the increased intake of food, many hyperthyroid cats lose weight. Vomiting is a common symptom with hyperthyroidism, owing to the direct overstimulation of the gastrointestinal tract and the often large increase in food intake. Less common symptoms include increased thirst and urine output, diarrhea and hyperactivity. There is also a small subset of cats (about 10%) which develop hyperthyroidism who have an atypical response, with lethargy, poor appetite and weakness.

Diagnosing hyperthyroidism is relatively simple. A good history and physical exam combined with a blood sample to confirm elevated thyroid level will often verify the condition. It is important to remember, however, that many of the symptoms of hyperthyroidism such as weight loss, appetite alterations, changes in behaviour, thirst and urination overlap a number of other conditions found in senior cats such as diabetes, kidney failure, inflammatory bowel and even cancer. For this reason, it is often best to screen multiple organ systems with a comprehensive blood sample (and often a urine analysis), so that all potential problems are revealed.

Treatment of hyperthyroidism is accomplished via either lifelong medication (methimazole) to “block” the production of thyroid hormone or radioactive Iodine can be given at a specialty hospital in order to destroy the thyroid nodules permanently.

Written by: Dr. S. Longridge