Introduction/Background Struma ovarii is a specialized or monodermal teratoma predominantly composed of mature thyroid tissue. Struma ovarii comprise 1% of all ovarian tumors and 2 to 5% of ovarian teratomas. It is most common between the ages of 40 and 60 years. We present an interesting case of ovarian serous cystadenoma with monodermal teratoma, without clinical or biochemical features of hyperthyroidism, as well as the therapeutic approach.

Methodology A 52 year-old-female, with personal history of polycystic ovary syndrome, multiple micro-nodular goiter and right oophorectomy due to a dermoid cyst of the right ovary, was admitted to the hospital due to continuous low abdominal pain. Blood parameters were within normal range. TSH was 2.5 IU/ml (n,v. 0.27–4.2). An abdominal sonography was performed and was revealed a heterogenous large mass in the left ovary, diameter 8 cm approximately, full of clear liquid. A Computed tomography was performed for the preoperatively assessment. A total hysterectomy with unilateral salpingo-oophorectomy was performed.

Results The histology of the ovary showed a large serous cystadenoma (size 8.5 × 5.5 cm). Nearby to the tumor was a segment with hyperplastic thyroid follicles and colloid abundance compatible to monodermal teratoma (struma ovarii type) with benign histological features.

Conclusion Struma ovarii is a rare ovarian tumor mainly with benign histological features. When the cause of hyperthyroidism is not obvious, struma ovarii could be a possible cause.

Disclosure Nothing to disclose.