A Giant Ectopic Parathyroid Adenoma Mimicking Parathyroid Carcinoma

Objective: Primary hyperparathyroidism (PHPT) is most caused by a solitary parathyroid adenoma and typically presents with mild-to-moderate hypercalcemia. Parathyroid carcinoma (PC), though rare, is suspected when hypercalcemia is severe, parathyroid hormone levels are markedly elevated, and there is a large neck mass. We present a case of a giant ectopic parathyroid adenoma causing severe hypercalcemia, highlighting the clinical overlap between giant adenomas and PC. Case Report: A 74-year-old woman with long-standing PHPT due to an ectopic mediastinal parathyroid mass, presented with a hypercalcemic crisis. Imaging showed a growing mediastinal mass measuring 5.5 × 3.0 cm. Given the rapid clinical deterioration, severe hypercalcemia, and tumor size, PC was suspected. The patient was stabilized with intravenous fluids and a single dose of denosumab. Surgical resection revealed a 6.0 cm, 14.2 gram ectopic parathyroid adenoma with no evidence of malignancy. Discussion: PC carcinoma is a rare cause of PHPT. Compared to parathyroid adenoma, it affects men and women equally and usually presents at a younger age. Clinically, it manifests with severe hypercalcemia, and target organ damage is frequently seen. A rare subset of patients with parathyroid adenoma can present a large parathyroid mass, referred as atypical or giant adenomas, that manifest similarly to PC. Conclusion: Giant parathyroid adenomas can produce extreme biochemical abnormalities and mimic carcinoma. Early multidisciplinary evaluation and tailored preoperative management are critical in such complex presentations.