An underactive thyroid, also known as hypothyroidism, is a condition in which your thyroid gland doesn't produce enough hormones. This can result from various factors such as autoimmune disease, iodine deficiency, or damage to the gland due to surgery or radiation therapy. Symptoms of an underactive thyroid include fatigue, weight gain, sensitivity to cold, constipation, dry skin, hair loss, and depression. Treatment typically involves hormone replacement therapy with synthetic thyroxine (levothyroixne) to help regulate the body's metabolism and alleviate symptoms.