A teratoma is a type of tumor that derives from pluripotent germ cells. The word comes from a Greek term meaning roughly "monster tumor". Teratomas (more correctly teratomata) usually start from cells in the testes in men, the ovaries in women and in the sacrum in children. Teratomata involve cells from all three embryonic cell layers: ectoderm, mesoderm and endoderm. They can be benign or malignant.

Teratomata often contain well-differentiated cells which can result in tissues growing in a teratoma which are quite different to the surrounding tissue—ovarian teratomas have been known to grow hair and teeth. Such a benign cystic teratoma is often termed a dermoid cyst, nowadays more correctly termed a mature teratoma. Some teratomata may contain a mixture of well-differentiated, mature tissues as: respiratory epithelium, hair follicle, fat tissue or mature nervous tissue [1]. Immature teratomata of the ovary have a malignant potential in line with the amount of neuroblastic tissue present.

Testicular teratomata are generally less well-differentiated, and have a worse prognosis (chances of recovery are not as high).

Some teratomata secrete the "pregnancy hormone" human chorionic gonadotropin (HCG), which can be used in clinical practice to follow-up successful treatment or relapse in patients with a known HCG-secreting teratoma. It is not recommended as a diagnostic marker.

Some teratomata secrete thyroxine, in some cases to such a degree that it can lead to clinical hyperthyroidism in the patient.

Struma ovarii

Struma ovarii (literally: goiter of the ovary) is a rare specialized type of teratoma present in the ovary that contains benign thyroid tissue. To be classified as a struma ovarii, thyroid tissue must be the predominant histology. Malignant transformation of struma ovarii is rare, occurring in only 5% of cases.