Follicular Variant Papillary Thyroid Carcinoma Arising in Struma Ovarii with a Concurrent Oncocytic Neoplasm

Background Malignant struma ovarii is a rare form of ovarian teratoma defined by the presence of malignant thyroid tissue, most frequently observed in middle-aged women. Discovery during pregnancy, particularly with mixed thyroid histology and atypical immunophenotypic features, is uncommon. This report describes a patient diagnosed with malignant struma ovarii during pregnancy that exhibited invasive follicular variant papillary thyroid carcinoma and a concurrent oncocytic neoplasm of uncertain malignant potential. Case Presentation A 27-year-old woman underwent routine obstetric ultrasonography at 15 weeks and 5 days of gestation, which identified a right ovarian cyst presumed to be a dermoid cyst. Cystectomy performed during cesarean section revealed a mature cystic teratoma containing a 4-millimeter focus of malignant thyroid tissue, composed of infiltrative follicular variant papillary thyroid carcinoma and an associated oncocytic neoplasm with features suggestive of vascular invasion. No necrosis was observed, and proliferative activity was low. Molecular analysis detected no mutations. Synaptophysin positivity was present in approximately 65% of oncocytic cells, with rare calcitonin positivity. Thyroid ultrasound identified a low-suspicion 0.8-centimeter thyroid nodule. Positron emission tomography demonstrated no metastatic disease. Completion surgery consisting of right salpingo-oophorectomy with peritoneal biopsies revealed no residual malignancy. The patient was classified as having stage IA disease and elected surveillance. Reports of mixed thyroid neoplasms and atypical oncocytic components within malignant struma ovarii are rare and may complicate both pathological interpretation and clinical management. Conclusion This case highlights the diagnostic complexity of malignant struma ovarii presenting during pregnancy, particularly when mixed histology, atypical immunophenotypic findings, and absence of detectable driver mutations are present.