Hypothyroidism is defined as low thyroid levels which is almost always caused by an underactive thyroid gland but rarely can be caused by an underactive pituitary gland.  The most common cause of an underactive thyroid gland is Hashimoto’s disease which is an autoimmune disease affecting the thyroid gland.  Other less common causes of an underactive thyroid gland include damage to the thyroid from medications, infections, surgery and radiation.  Common symptoms of hypothyroidism are fatigue, weight gain, constipation, dry skin, cold intolerance, irregular menstrual cycles, mood changes and difficulties with memory and concentration.  Hypothyroidism is diagnosed by checking blood hormone levels and is treated with thyroid hormone replacement.