A 15-cm struma ovarii with intestinal obstruction misdiagnosed as a mesenteric cyst: a case report

Struma ovarii is a rare monodermal ovarian teratoma composed predominantly of thyroid tissue. Because its clinical and imaging features are non-specific, it is frequently misdiagnosed before surgery. Giant struma ovarii accompanied by torsion, necrosis, and intestinal obstruction is uncommon in young women. Ascites and mildly elevated CA-125 may mimic ovarian malignancy and pose diagnostic challenges. This case highlights diagnostic pitfalls when the disease first presents to a general surgery service. A 26-year-old multiparous woman presented with intermittent right-sided dull abdominal pain for two months and a palpable abdominal mass, followed by worsening pain, fever, and incomplete intestinal obstruction. Laboratory tests showed a marked inflammatory response and mildly elevated CA-125. On preoperative computed tomography the 164 × 122 × 153 mm cystic mass was misinterpreted as a mesenteric cyst; transvaginal ultrasound suggested torsion of an ovarian cyst. Exploratory laparotomy revealed a giant left ovarian cyst that was torsed and necrotic, with 300 mL of ascites. Fertility-sparing surgery was performed in the form of a left partial oophorectomy. Intraoperative frozen section suggested a benign lesion; final paraffin histology confirmed struma ovarii (thyroid follicles approximately 55%, no other germ cell elements). The patient had an uncomplicated postoperative course. At the 6-month follow-up, CA-125 had returned to normal and no recurrence has been detected to date. Giant struma ovarii with torsion and intestinal obstruction may be misdiagnosed as other intra-abdominal cystic lesions, such as a mesenteric cyst. For young women with a large abdominal cystic mass, acute abdominal symptoms and mildly elevated CA-125, struma ovarii should be included in the differential diagnosis. Enhanced MRI may help identify thyroid-tissue-related imaging features. Intraoperative frozen section examination is important to prevent overtreatment and safeguard fertility; long-term surveillance is advised for benign struma ovarii.