The most common cause of hypothyroidism is Hashimoto's thyroiditis. Thyroiditis is an inflammation of the thyroid gland. Hashimoto's thyroiditis is an autoimmune disorder. With Hashimotoâs, your body produces antibodies that attack and destroy the thyroid gland.Thyroiditis may also be caused by a viral infection.ypothyroidism, also called underactive thyroid disease, is a common disorder. With hypothyroidism, your thyroid gland does not make enough thyroid hormone. The thyroid gland is located in the front lower part of your neck.