The thyroid is a gland in the neck that produces hormones that help to regulate how the body uses energy. Hyperthyroidism is a disease in which the thyroid makes too much thyroid hormone, speeding up many of the body's functions. Symptoms of hyperthyroidism include rapid heartbeat, feeling hot, anxiety, muscle weakness, frequent bowel movements, trouble sleeping, shakiness, light or missed menstrual periods, and weight loss. Hypothyroidism is a condition in which the thyroid makes too little thyroid hormone and body functions slow down. Symptoms of hypothyroidism include tiredness, feeling cold, constipation, hoarse voice, changes in hair and skin, heavy menstrual periods, and weight gain. Doctors diagnose thyroid disease by checking blood levels of thyroid hormones. Thyroid disease can usually be easily treated with drugs; radiation; or, occasionally, surgery.