Thyroid disease is a medical condition that affects the thyroid, a gland located in the neck that produces hormones that control many of the body's functions. Symptoms of thyroid disease can include fatigue, weight gain or loss, depression, anxiety, and changes in heart rate or blood pressure. Common causes of thyroid disease include autoimmune diseases, medications, radiation therapy, and iodine deficiency. Diagnosis of thyroid disease usually involves a physical examination, blood tests, and imaging tests. Treatment of thyroid disease usually involves hormone replacement therapy or surgery, depending on the type and severity of the condition.