Hypothyroidism is a condition in which the thyroid gland does not produce enough thyroid hormone, which regulates the way the body uses energy.

A low thyroid level can cause symptoms of fatigue, weakness, lethargy, weight gain, depression, memory problems, constipation, dry skin, intolerance to cold, coarse and thinning hair, brittle nails, or a yellowish tint to the skin.

Hypothyroidism usually develops slowly. It often results from a problem in which the body's own natural defense (immune) system attacks the thyroid gland (autoimmune thyroiditis or Hashimoto's thyroiditis). Hypothyroidism may also develop following radiation treatment or surgical removal of the thyroid.

Hypothyroidism is treated with medications to replace the thyroid hormone. Symptoms usually disappear within a few months after treatment begins, but most people need to continue taking thyroid hormones for life.