Benign struma ovarii in a patient with persistent thyroglobulin elevation after papillary thyroid carcinoma in the era of RAI de-escalation

Struma ovarii (SO) is a rare mature ovarian teratoma composed predominantly of thyroid tissue. In patients with thyroid cancer, distinguishing benign SO from metastatic thyroid carcinoma can pose a substantial diagnostic challenge. A 51-year-old woman with T1bN0 papillary thyroid carcinoma developed persistently elevated serum thyroglobulin (Tg) levels after total thyroidectomy, ranging from 37.9 to 45.8 ng/mL, despite negative findings on neck and chest imaging. Positron emission tomography-computed tomography and pelvic magnetic resonance imaging revealed a 7.2-cm multiloculated cystic lesion in the left ovary, and pathological examination confirmed struma ovarii. After total abdominal hysterectomy with bilateral salpingo-oophorectomy, serum Tg levels normalized and remained undetectable throughout a 2-year surveillance period. Serum Tg may serve as an important biomarker for detecting functional ectopic thyroid tissue, including SO. In the contemporary era of de-escalated radioactive iodine therapy for low-risk thyroid cancer, ectopic thyroid tissue should be considered in patients with unexplained persistent Tg elevation and no evidence of recurrence on standard surveillance. In such cases, clinically guided abdominal or pelvic imaging may facilitate timely identification of ectopic thyroid tissue.