Jialin Pan, Bowen Tan, Xueyun Zhang
Clinicians should consider IHNPC in the differential diagnosis of any thyroid nodule, especially those with ill-defined borders or abnormal PTH levels, even in the absence of hypercalcemia. Routine measurement of serum PTH and 25-hydroxyvitamin D in patients with thyroid nodules, along with multimodal imaging and intraoperative frozen section when indicated, is essential to unmask this diagnostic masquerader and guide appropriate management.
BACKGROUND: Intrathyroidal hypofunctional normocalcemic parathyroid carcinoma (IHNPC) is a rare and aggressive malignancy; while only a small number of well-documented cases have been reported in the literature, multiple case series have described similar presentations of intrathyroidal parathyroid carcinomas (PCs) presenting as thyroid nodules without hypercalcemia. This entity accounts for an extremely small fraction of parathyroid neoplasms, with only sporadic case reports documented globally. Characterized by minimal secretion of parathyroid hormone (PTH) resulting in mild PTH elevation without corresponding hypercalcemia and normal serum calcium concentration, this neoplasm poses a distinct diagnostic challenge by virtue of its intrathyroidal location, where its clinical and radiological features closely mimic those of a common thyroid nodule, often resulting in substantial diagnostic delays.
CASE DESCRIPTION: A 51-year-old woman presented with a solitary thyroid nodule that had been incidentally discovered two years earlier and monitored conservatively, and an isolated mild elevation of PTH. Imaging studies revealed a solid nodule with ill-defined margins within the right thyroid lobe. Initially misdiagnosed with a benign thyroid lesion by preoperative fine-needle aspiration cytology, she underwent right thyroid lobectomy. Intraoperative suspicion and subsequent definitive histopathology confirmed the diagnosis of PC. The patient recovered uneventfully, and at six-month follow-up, serum PTH and calcium levels had normalized with no evidence of recurrence.
CONCLUSIONS: Clinicians should consider IHNPC in the differential diagnosis of any thyroid nodule, especially those with ill-defined borders or abnormal PTH levels, even in the absence of hypercalcemia. Routine measurement of serum PTH and 25-hydroxyvitamin D in patients with thyroid nodules, along with multimodal imaging and intraoperative frozen section when indicated, is essential to unmask this diagnostic masquerader and guide appropriate management.