Monodermal, highly specialized mature cystic teratoma comprised of ectopic thyroid tissue

Struma ovarii is an extremely rare condition. It is defined as the presence of thyroid tissue comprising greater than 50% of the cellular component in an ovarian tumor, virtually always a teratoma. Struma ovarii is a mature teratoma and accounts for approximately 3% of all ovarian teratomas. It is usually benign, although 5% have malignant components that can occasionally metastasize. Tumors may have features of a multinodular goiter, with colloid nodules and hyperplastic changes. These tumors can vary in size, but most are greater than 5 cm at diagnosis.

The typical sonographic appearance of struma ovarii is similar to that of a dermoid cyst with one or more echogenic nodules known as struma pearls. Although the echogenic nodules in dermoids have no evidence of color flow on Doppler interrogation, the struma pearl may be quite vascular, which is a valuable clue to the correct diagnosis. Sonographically, most cases of struma ovarii are nonspecific in appearance and are largely solid or have both cystic and solid portions. Less commonly, the tumor is predominantly or entirely cystic, although most of these are multilocular. Occasionally, struma ovarii will have a unilocular cystic appearance, making a specific sonographic diagnosis difficult.