SAT-806 - Giant Parathyroid Adenoma Mimicking Parathyroid Carcinoma in Severe Primary Hyperparathyroidism with Prolonged Hungry Bone Syndrome

Background:
Giant parathyroid adenomas, defined as lesions weighing >3.5 g, are a rare cause of primary hyperparathyroidism and may mimic parathyroid carcinoma due to severe biochemical abnormalities, skeletal involvement, and invasive-appearing imaging or operative findings. Distinguishing benign from malignant disease preoperatively remains challenging and impacts surgical planning and postoperative management.

Case Presentation:
A 32-year-old male with no significant medical history presented after incidental discovery of severe hypercalcemia during evaluation for foot pain. Laboratory evaluation revealed serum calcium 14.6 mg/dL, alkaline phosphatase 867 U/L, and markedly elevated parathyroid hormone (PTH) levels rising from 1,375 pg/mL to 3,019 pg/mL. Vitamin D 25-hydroxy level was deficient at 11 ng/mL, while metanephrines, calcitonin, and prolactin were normal.
Parathyroid SPECT demonstrated persistent uptake in the left mid-to-upper thyroid region, and ultrasound revealed a 2.9 cm hypoechoic left upper pole lesion. FDG PET-CT showed a radiotracer-avid left parathyroid lesion (SUV max 11.5) with diffuse skeletal uptake and lytic bone changes, raising concern for parathyroid carcinoma with metastatic disease versus osteitis fibrosa cystica.
The patient was treated with intravenous fluids, zoledronic acid, glucocorticoids, and cinacalcet before undergoing en bloc resection of a large left superior parathyroid mass with left hemithyroidectomy and central neck dissection. Operative findings showed adherence to the esophagus with recurrent laryngeal nerve effacement but no gross invasion. Intraoperative PTH declined from 2,146 pg/mL to 190 pg/mL.

Pathology and Outcome:
Histopathology demonstrated a well-circumscribed, encapsulated parathyroid tumor measuring 3.2 cm and weighing 4.3 g, without cytologic atypia, mitoses, or capsular or vascular invasion. Ki-67 showed a low proliferative index, confirming a giant parathyroid adenoma.
Postoperatively, PTH decreased to 118 pg/mL at one week and 83 pg/mL at two weeks, followed by gradual elevation (173–232 pg/mL) with maintained normocalcemia, consistent with postoperative rebound. The patient developed severe hungry bone syndrome requiring prolonged intravenous and high-dose oral calcium and calcitriol, with supplementation continued at six months. Bone pain resolved, and alkaline phosphatase improved to 209 U/L.

Conclusion:
This case highlights the substantial overlap between giant parathyroid adenoma and parathyroid carcinoma. Diffuse skeletal FDG uptake and lytic lesions may represent osteitis fibrosa cystica rather than metastatic disease, underscoring the need for careful interpretation and anticipation of prolonged postoperative calcium requirements.

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