Hypothyroidism is often caused by inflammation of the thyroid gland. The most common type of thyroid inflammation is caused by an autoimmune disorder (chronic autoimmune thyroiditis). Another autoimmune disorder, called Graves disease, is a main cause of hyperthyroidism. In autoimmune disorders, the body's immune system mistakenly attacks its own healthy tissue with antibodies. No one understands why the body makes antibodies against thyroid tissue. Some people believe that cells called lymphocytes, passing from the blood into the thyroid gland, come in contact with substances (antigens) in the gland that trigger production of antibodies against thyroid tissue. A good test of this concept would be to see whether the body produces antibodies against thyroid tissue after removal of the thyroid gland and its antigens.