Introduction/Background Struma ovarii is a mature teratoma in which thyroid tissue predominates in the tumor histology. It is a rare tumor, responsible for approximately 1–5% of teratoma. Can be classified as benign or malign. The incidence is between 40 and 60 years old.

Methodology A case report of a malignant Struma Ovarii in a post-menopause woman.

Results Adnexal mass that grew over 11 months. On physical examination in June 2023, a pelvic mass was palpable approximately 10cm from the pubic symphysis, mobile and painless. In January 2022, a transvaginal ultrasound showed a right ovarian volume of 220.9cm3 with a cyst with homogeneous content measuring 6.2cm. In December 2022, a new ultrasound revealed a right ovary with 427.4cm3 containing a cyst measuring 11.3cm.

The tumor markers (CA 19–9: 32.4, CEA: 3.31, CA 125: 26.7) and thyroid function were normal.

In July 2023, a Exploratory Laparotomy with Total abdominal hysterectomy and bilateral salpingo-oophorectomy was performed with the finding of an adnexal mass measuring 10cm, with a smooth surface and a solid-cystic component, without other abnormalities in the abdominal cavity. The intraoperative frozen section biopsy, suggested a serous cystadenoma. Final pathology demonstrated papillary microcarcinoma originating in the Struma ovarii area, adjacent thyroid parenchyma with nodular follicular hyperplasia and Hurthle cell metaplasia, adjacent ovarian parenchyma with serous cystadenoma.

Patient was referred for clinical oncology evaluation, without indication of adjuvant therapy. Patient maintains multidisciplinary follow-up, asymptomatic, euthyroid and with no evidence of disease.

Conclusion Struma ovarii is a rare teratoma and It is usually asymptomatic or oligosymptomatic. There is generally no impact in thyroid function, however there are reports in the literature of patients developing hyperthyroidism or hypothyroidism after surgical resection.

Disclosures The Treatment consists of surgical resection of teratoma, while the assessment of adjuvant therapy depends on its histopathological characteristics.