Struma ovarii

A struma ovarii (literally: goitre of the ovary) is a rare form of monodermal teratoma that contains mostly thyroid tissue, which may cause hyperthyroidism.Despite its name, struma ovarii is not restricted to the ovary. The vast majority of struma ovarii are benign tumours; however, malignant tumours of this type are found in a small percentage of cases. [Source: Wikipedia ]