Struma Ovarii
A rare teratoid tumor of the ovary composed almost entirely of thyroid tissue, with large follicles containing abundant colloid. Occasionally there are symptoms of hyperthyroidism. 5-10% of struma ovarii become malignant, the only absolute criterion for which is the presence of metastasis. (Dorland, 27th ed; Segen, Dictionary of Modern Medicine, 1992)