In hypothyroidism, the thyroid gland does not produce enough thyroid hormones. These hormones regulate metabolism (chemical reactions that keep the body functioning), and insufficient amounts cause body processes to slow down. This produces a range of symptoms, including fatigue, weight gain, dry skin, mental slowness, and sometimes enlargement of the thyroid (goiter). Babies with hypothyriodism may have feeding difficulties and jaundice (yellowing of the skin and eyes).
Hypothyroidism is most common in adults and is usually due to an autoimmune disorder in which the immune system attacks thyroid tissue, although it may also be caused by the removal of thyroid tissue to treat hyperthyroidism (overactivity of the thyroid, see left).
Rarely, babies are born with an underactive thyroid. Treatment is with replacement thyroid hormone.