An under active thyroid is when the thyroid does not produce enough thyroid hormone, the affects are largely opposite to that of an over active thyroid. The metabolism slows down, these is a degree of weight gain, feeling of coldness and the heart rate slows down. This condition is also known as Myxoedema and can be caused by antibodies against the thyroid gland. The treatment is investigative blood tests to get the level of thyroid hormones and the treatment is thyroid replacement with Thyroxin.