Struma ovarii is a teratoma of the ovaries that contains a large amount of thyroid tissue. Like the cervical thyroid gland, this ectopic thyroid tissue can become autonomous. We present a case of hyperthyroidism caused by thyroid tissue in a large ovarian cystic teratoma and provide detailed endocrinological, radiological, and pathological preoperative and postoperative data. This is also the first documented case of struma ovarii in association with a secreting pituitary tumor. In addition, we provide a retrospective pathological analysis of 1390 surgically removed ovarian tumors at 2 major academic centers.