Hypothyroidism is a condition in which the thyroid gland does not produce enough of the hormone thyroxine. It can result in a variety of symptoms, such as fatigue, depression, weight gain, dry skin, and sensitivity to cold temperatures. The most common cause of hypothyroidism is an autoimmune disorder known as Hashimoto's thyroiditis. Other causes include medications, radiation, and iodine deficiency. Treatment typically involves taking daily thyroid hormone replacement medication. If left untreated, hypothyroidism can lead to serious health complications.