The most common cause of hyperthyroidism is Graves’ disease. Graves’ disease is an autoimmune disorder in which antibodies produced by the immune system stimulate the thyroid to produce too much thyroxine. In Graves’ disease, antibodies mistakenly attack the thyroid gland and occasionally the tissue behind the eyes and the skin of the lower legs. Some people with Graves’ disease have exoph-thalmos, a bulging of the eyes associated with Graves’ disease. Treatment options include antithyroid drugs, radioactive iodine, or surgery. Radioactive iodine is the most common treatment for hyperthyroidism. Some thyroid cells will absorb the radioactive iodine. After a period of several weeks, the cells that took up the radioactive iodine will shrink and thyroid hormone levels will return to normal.