Hypothyroidism is a condition in which the thyroid gland is not able to produce enough thyroid hormone. Since the main purpose of thyroid hormone is to "run the body's metabolism," it is understandable that people with this condition will have symptoms associated with a slow metabolism. Hypothyroidism means that the thyroid gland can't make enough thyroid hormone to keep the body running normally. People are hypothyroid if they have too little thyroid hormone in the blood. Common causes are autoimmune disease, such as Hashimoto's thyroiditis, surgical removal of the thyroid, and radiation treatment.