The thyroid gland is an endocrine gland that secretes hormones that regulate the metabolism. Hypothyroidism is an underactive thyroid gland. This is due to congenital reasons, age, treatment of an overactive thyroid with radiation, or even a respiratory illness that could damage the thyroid. Symptoms include–fatigue, depression, thinning hair, weight gain, brittle nails, heavy menstrual periods, among others. Treatment is hormone replacement to supply the body with thyroxine which will usually be for life. Levels must be checked to be sure that enough thyroid hormone is present. Myxedema coma is a life-threatening symptom that occurs in untreated hypothyroidism. Other complications can be infertility and heart disease.