Concurrent papillary thyroid carcinoma, normocalcemic primary hyperparathyroidism, and incidental struma ovarii in a young woman: a case report.

Introduction and importance: Struma ovarii is a rare monodermal ovarian teratoma composed predominantly of thyroid tissue, accounting for approximately 1% of ovarian neoplasms and 2%-3% of mature teratomas. Its coexistence with differentiated thyroid carcinoma - and particularly in the setting of normocalcemic primary hyperparathyroidism - has not been previously described. Case presentation: A 26-year-old woman presented with a painless 2 × 2 cm midline neck swelling. Ultrasound and fine-needle aspiration confirmed papillary thyroid carcinoma. Biochemistry revealed an elevated parathyroid hormone (81.1 pg/mL) despite normal serum calcium (9.4 mg/dL) and vitamin D insufficiency. Tc-99m sestamibi single-photon emission tomography with computed tomography (SPECT-CT) localized a left inferior parathyroid adenoma. She underwent total thyroidectomy, central lymph node dissection, and parathyroidectomy. Forty-five days later, whole-body I-131 scintigraphy for remnant ablation revealed unexpected pelvic uptake. SPECT-CT and transvaginal ultrasound identified a 2.6 × 2.5 cm left ovarian cystic mass. Tumor markers were normal. Laparoscopic cystectomy was performed. Histopathology confirmed benign struma ovarii arising within a mature cystic teratoma. The patient's postoperative course was uneventful, she resumed thyroid hormone replacement, and follow-up showed normalization of parathyroid hormone levels without recurrence. Clinical discussion: Concurrent presentation of papillary thyroid carcinoma, normocalcemic primary hyperparathyroidism, and struma ovarii constitutes a novel endocrine triad not previously described. Normocalcemic primary hyperparathyroidism requires a high index of suspicion and rigorous biochemical and imaging evaluation, since normal serum calcium may mask a significant parathyroid adenoma. Pelvic radioiodine uptake outside the thyroid bed can also be seen in benign struma ovarii, not only metastasis, underscoring the need to correlate functional and anatomical imaging. Accurate diagnosis and tailored management of such complex endocrine neoplasms depend on a multidisciplinary approach and strict long‑term surveillance. Conclusion: This report provides important educational insights for endocrinologists and surgeons managing concurrent endocrine pathologies. Normocalcemic primary hyperparathyroidism may mask a significant parathyroid adenoma and requires high clinical suspicion despite normal serum calcium. Critically, in thyroid cancer surveillance, unexpected extra-thyroidal radioiodine uptake - particularly in the pelvis - must include struma ovarii in the differential diagnosis to prevent inappropriate treatment escalation. Multimodal imaging and thorough histopathological evaluation are essential to distinguish benign struma ovarii from metastatic disease and guide appropriate management.