At a Glance

Struma ovarii usually refers to the presence of significant amount of thyroid tissue in ovarian tumors, typically monodermal teratomas. Although a small amount of thyroid tissue may be present in up to 15% of ovarian teratomas, in only 1-2% of teratomas does thyroid tissue comprise more than 50% of the teratoma: thus the name struma ovarii. These usually present in the fifth decade of life; most are benign, although less frequently (5-37%) teratomas show malignant transformation; and 10% present with metastasis.

Some (5%) patients with struma ovarii present with symptoms of hyperthyroidism, absence of thyroid gland enlargement, an elevated thyroglobulin level, and a low thyroid radioiodine uptake. More commonly, women present with symptoms of a one-sided pelvic mass that may be associated with ascites and/or pleural effusion, as well as elevation of CA-125 levels in some cases.

What tests should I request to confirm my clinical Dx? In addition, what follow-up tests might be useful?

Diagnosing struma ovarii is less difficult when the patient presents with symptoms and laboratory findings of hyperthyroidism in the absence of thyroid enlargement and absence of increased radioiodine uptake (RAIU) in the thyroid gland. In such instances, TSH, FT4, thyroglobulin, and a low radio-iodine uptake in the thyroid are useful (Table 1).

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Table 1.
TSH FT4 Thyroglobulin
Low High Elevated

Ultrasound examination of the ovaries, as well as CT and MR studies, can help identify multilocular cystic ovarian masses with a solid component, but these do not allow differentiation of struma ovarii from other ovarian masses. To confirm, it is necessary to document a hyperfunctioning struma ovarii based on the higher radioiodine uptake in the struma ovarii as compared to the thyroid gland. Pathologic examination of tissues is critical in confirming the diagnosis and differentiating benign from the malignant struma ovarii.

Are there any factors that might affect the lab results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

In pseudo-meigs syndrome (ovarian teratoma, ascites, pleural effusion), elevated CA-125 can suggest an ovarian malignancy (60% positive predictive value), but it is important to recognize that an elevated CA-125 is also known to occur in benign struma ovarii.

What Lab Results are Absolutely Confirmatory?

Scintigraphy showing increased radioiodine uptake in the pelvic mass compared to the thyroid is confirmatory. Histologic examination of tissues is a must to evaluate the extent of thyroid tissue in the teratoma, as well as for diagnostic purposes and differentiation from other types of ovarian tumors.

Additional issues of Clinical Importance

It is important to consider struma ovarii in a woman with a proposed laparotomy because a thyroid crisis can be precipitated following the surgical procedure. Alternately, hypothyroidism is also possible following removal of the ovarian tumor. Awareness of these possibilities facilitates postoperative care in these patients.

Errors in Test Selection and Interpretation

As discussed, elevation of CA-125 is not diagnostic of a malignant ovarian tumor and is known to occur in benign struma ovarii.

Rarely, Grave’s disease or a toxic adenoma can coexist with struma ovarii. This complicates radioiodine uptake studies, which show an increased uptake in both the thyroid and the struma ovarii.

A struma carcinoid, although very rare, should not be missed on histologic examination. It is characterized by the presence of both carcinoid and thyroid tissue in the struma ovarii. Clinical manifestations include hyperandrogenism or hyperestrogenism, as well as hyperthyroidism.

Histologic entities that need to be differentiated by pathologic examination of tissue include other primary ovarian carcinomas, sertoli-leydig cell tumors, and metastatic cancers to the ovary.