There are two fairly common causes of hypothyroidism. (1) The first is a result of previous (or currently ongoing) inflammation of the thyroid gland which leaves a large percentage of the cells of the thyroid damaged (or dead) and incapable of producing sufficient hormone. The most common cause of thyroid gland failure is called autoimmune thyroiditis (also called Hashimoto's thyroiditis), a form of thyroid inflammation caused by the patient's own immune system. (2) The second major cause is the broad category of "medical treatments". The treatment of many thyroid conditions warrants surgical removal of a portion or all of the thyroid gland. If the total mass of thyroid producing cells left within the body are not enough to meat the needs of the body, the patient will develop hypothyroidism. (3) There are several other rare causes of hypothyroidism, one of them being a completely "normal" thyroid gland which is not making enough hormone the pituitary gland does not produce enough thyroid stimulating hormone (TSH). Then the thyroid simply does not have the "signal" to make hormone.
Women are three times more likely than men to have an underactive thyroid. In fact, as many as ten percent of women may have some degree of thyroid hormone deficiency.
Over five million Americans have some degree of hypothyroidism. Hashimoto's thyroiditis, an autoimmune disease that is the most common cause of hypothyroidism in the USA. This condition is characterized by inflammation and damage to the thyroid tissue.