A Giant Ectopic Parathyroid Adenoma Mimicking Parathyroid Carcinoma

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.

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.

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.

Giant parathyroid adenomas can produce extreme biochemical abnormalities and mimic carcinoma. Early multidisciplinary evaluation and tailored preoperative management are critical in such complex presentations.