Hypothyroidism is a hormonal disorder in which the thyroid gland fails to produce an adequate amount of thyroid hormone. The thyroid gland is responsible for many things such as regulating metabolism of many body systems. Those often affected are the skin/hair coat, gastrointestinal, neurologic, reproductive, and cardiovascular. Hypothyroidism is a condition that typically affects medium to larger breed dogs. Cats rarely get hypothyroidism. Their condition is an over active thyroid gland, or hyperthyroidism.  The most common cause of hypothyroidism is an immune complex targeting the thyroid gland.

Clinical signs of hypothyroidism vary between dogs but more commonly exhibit weight gain (despite a usual lack of appetite), hair loss (especially on their tail--"rat tail"), and mentally dull and lethargic. Due to many other vague clinical signs, hypothyroidism should always been on a differential diagnosis list, especially if weight gain is occurring.

Diagnosis is possible with a clinical suspicion of the disease as well as a thorough history and physical exam. The definitive diagnosis is through blood tests that include the more specific Free T4 as well as Total T4. If diagnosis is not as straightforward as anticipated, a third blood value, the TSH, may be necessary.  

Treatment involves replacing the lacking thyroid hormone with a once or twice a day medicine. Therapy is usually life long. Annual blood tests are necessary to determine the appropriate dosage.