Intrathyroidal parathyroid carcinoma in a patient with multiple endocrine neoplasia type 1

Primary hyperparathyroidism occurs in up to 95% of patients with multiple endocrine neoplasia type 1 (MEN1). However, only a few cases of parathyroid carcinoma have been reported, and only one case of intrathyroidal parathyroid carcinoma. We describe the case of a 41-year-old male with a history of MEN1 and primary hyperparathyroidism who developed hypercalcemia to 13.1 mg/dL (SI: 3.3 mmol/L) (reference range [RR], 8.6-10.3 mg/dL [SI: 2.2-2.6 mmol/L]) with parathyroid hormone (PTH) 454 pg/mL (SI: 48.1 pmol/L) (RR, 15-65 pg/mL [SI: 1.6-6.9 pmol/L]). Neck ultrasonography showed a highly suspicious intrathyroidal 3.6 cm nodule. The patient had a lobectomy, and final pathology showed a 3.1 × 1.6 × 1.2 cm parathyroid carcinoma. After the surgery, calcium dropped to 6.0 mg/dL (SI: 1.5 mmol/L) with PTH of 11 pg/mL (SI: 1.2 pmol/L) requiring oral calcium supplements, calcitriol, and hydrochlorothiazide. Laboratory tests at 12 months on these medications showed calcium of 9.3 mg/dL (SI: 2.3 mmol/L) and PTH 21 pg/mL (SI: 2.2 pmol/L) indicating recovery of parathyroid function without recurrence, and calcitriol was discontinued. This case illustrates that a high index of suspicion for malignancy should be maintained for intrathyroidal parathyroid adenoma in the setting of severe hypercalcemia, especially in MEN1.