Hypothyroidism is a disease that results from a lack of thyroid hormone production by the thyroid gland. The thyroid gland is a small gland located at the throat, near what might be termed in people the “adam’s apple”. Hypothyroidism is thought to be cause by an immune mediated process within the thyroid gland or idiopathic atrophy of the gland – uncommonly it can be congenital. It is seen most commonly in middle aged to older dogs. There is no sex predilection and occurs mostly in medium to large breed dogs.
The thyroid gland secretes thyroid hormones that affect the metabolism of every cell in the body and therefore hypothyroidism can have many systemic effects. Clinical signs include dogs that become easily fatigued, sleep for longer periods of time, are often dull in their attitude, are prone to weight gain, and may seek a warm area in cold weather. The hair coat becomes dry and loses sheen, the color dulls and the hair may become very sparse, especially in areas of friction, with darkening of the skin. Clipped hair may not regrow well. Dogs may become prone to skin infections and poor wound healing. Because thyroid hormones affect the metabolism of all cells throughout the body, other systemic effects can be seen as well including cardiovascular, neurologic, and gastrointestinal signs. Often times dogs may not show any obvious clinical signs to owners and the disease is picked up incidentally on routine bloodwork.
Diagnosis of hypothyroidism involves taking one to several blood tests to measure levels of thyroid hormones. Tests may include measuring total T4, free T4, and/or TSH. In some cases, diagnosis can be difficult and repeated testing is necessary as the thyroid levels can be affected by non-thyroidal illness.
Treatment involves administration of oral thyroid hormone supplementation twice daily. Once thyroid supplementation is started, thyroid levels are checked every one to three months until the levels are normal. Once stabilized, thyroid hormone levels are routinely checked every 6-12 months. There is generally a rapid improvement in activity after initiating therapy. However, if the skin has been affected, regrowth of the hair and lightening of the skin may take several months. Prognosis is excellent but treatment is life-long as there is no true cure for hypothyroidism.