The 12 cm Masquerader: Struma Ovarii Disguised as a Benign Mucinous Cystadenoma (Ovarian-Adnexal Reporting and Data System 3) in a Young Woman.

Struma ovarii is a rare ovarian tumor that can clinically and radiologically resemble other benign adnexal masses, creating diagnostic challenges, particularly in reproductive-age women in whom fertility preservation is a priority. We report the case of a 30-year-old nulliparous woman who presented with a one-month history of pelvic fullness and a palpable lower abdominal mass. Imaging demonstrated a large multiloculated left ovarian cyst, interpreted as a benign lesion. Tumor markers were within normal limits, and the patient was clinically euthyroid. She underwent fertility-sparing left ovarian cystectomy with ovarian reconstruction, and the diagnosis of struma ovarii was established following surgical excision and histopathological examination. The postoperative course was uneventful. Struma ovarii should be considered in the differential diagnosis of complex ovarian masses, even when imaging and tumor markers suggest a benign etiology. Histopathological confirmation is essential for accurate diagnosis and management. In benign cases, fertility-sparing surgery is an effective treatment option.