Struma ovarii is a rare ovarian tumor that was first described in 1899. It is defined by the presence of thyroid tissue comprising more than 50% of the overall mass. It most commonly occurs as part of a teratoma, but may occasionally be encountered with serous or mucinous cystadenomas.  Strumae ovarii comprise 1% of all ovarian tumors and 2-5% of ovarian teratomas.
Several variants of the tumor exist. Benign strumosis is a rare version where mature thyroid tissue implants are present throughout the peritoneal cavity. Strumal carcinoid is defined by the presence of carcinoid tissue within a struma ovarii. The vast majority of strumae ovarii are benign, but malignant disease is found in a small percentage of cases, the most common being papillary thyroid carcinoma.
The symptoms of struma ovarii are similar to those of other ovarian tumors and are nonspecific in nature. The tumor can be characterized by imaging, but the final diagnosis is made by pathological and histological examination. Surgical resection remains the definitive treatment for benign disease, and surgery with adjuvant radioiodine therapy has been shown to be successful in treating metastatic and recurrent disease. 
Struma ovarii is rare. Approximately 1% of all ovarian tumors and 2.7% of all dermoid tumors are classified as struma ovarii. 
Malignancy is defined by various criteria in different studies, principally differing on classifying struma as either a thyroid or ovarian cancer. In the most recent World Health Organization classification, malignant struma ovarii are included in the thyroid tumor group  Several other types of tumors, such as Brenner tumor or cystadenoma, may also be found with a struma.
· Malignant change seems to occur in about a third of cases. 
· Metastatic spread, which follows the pattern of ovarian cancer, occurs in approximately 5% of malignant cases. 
· Survival rates are excellent. 
Although the tumor is predominately composed of thyroid tissue, thyrotoxicosis is seen in only 5% of all cases. Only 1 case of thyrotoxicosis resulting from peritoneal strumosis has been reported. 
In a study of 68 patients with malignant struma ovarii, Goffredo et al found excellent disease-specific survival rates for the condition no matter which treatment—unilateral oophorectomy, bilateral oophorectomy, oophorectomy and omentectomy, or debulking surgery—was used. The report, which utilized the Surveillance, Epidemiology, and End Results (SEER) database of the National Cancer Institute, also determined that the risk of aggressive thyroid cancer in patients with malignant struma ovarii is high. The overall 5-, 10-, and 20-year survival rates in the study were 96.7%, 94.3%, and 84.9%, respectively, with only one of the deaths that occurred being attributed to malignant struma ovarii. The investigators found, however, that six patients (8.8%) were diagnosed concomitantly or subsequently with thyroid cancer, with two thirds of the thyroid cancers growing beyond the thyroid gland. All of the thyroid cancer patients were still alive at the last follow-up. 
Because of its rarity, no clear racial predilection for struma ovarii has been determined.
Defined as a tumor of ovarian origin, struma ovarii occurs exclusively in genetic females.
See the list below:
Struma ovarii typically presents between the ages of 40-60.
Struma ovarii rarely occurs before puberty.