Canine hypothyroidism, or low thyroid function, is the most common hormonal disorder in dogs. The clinical signs are caused by low levels of thyroid hormones released by the thyroid gland. Commonly, dogs are tested by measuring the circulating thyroid hormone thyroxine or T4. Other additional tests help reduce the number of false positive and false negative test results. The most common reason for a false diagnosis of hypothyroidism is the presence of other illness. In a sick dog, T4 levels may be suppressed. Testing for "free T4" tests for the small amount of T4 hormone that's not bound to blood proteins. Illness does not affect the measurement of free T4 as much as T4.
The affects of low thyroid function can be seen throughout the body. These affects can appear quite slowly, as the dog's metabolic rate gradually decreases. Some of the most common signs include poor haircoat or hairloss, slow heart rate, infertility, lethargy, mental depression, or exercise intolerance. Hypothyroidism is easily treated. The missing hormone is available as a tablet, and giving adequate replacement orally will reverse the clinical signs. It can take weeks of medicating to recognize the improvement. Since the thyroid gland is not regaining function through the treatment, and treatment consists of providing the hormone that the thyroid gland is not able to provide, treatment must be life-long.
