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SAT-503 A Rare Case of Hypercalcemia Due to PTH Secreting Lung Adenocarcinoma

Abstract Introduction: Hypercalcemia can be present in up to 20% of malignancies. The majority are caused by humoral hypercalcemia of malignancy, mediated by secretion of PTHrP, typically by squamous cell carcinomas. Other etiologies of hypercalcemia in malignancy include local osteolysis, and increased production of 1,25(OH)2D. In rare cases, hypercalcemia can be caused by secretion of PTH by tumors. Clinical Case: A 60-year-old man with metastatic lung adenocarcinoma presented with altered mental status and was noted to have hypercalcemia to 19.2 mg/dL (reference range, 8.5-10.1). Imaging revealed a 12.0 cm right mediastinal soft tissue mass. He was initially treated with isotonic saline, calcitonin, and zolendronic acid. PTH was elevated to 594 pg/mL (reference range, 14-72), 25OHD was low at 13.2 ng/mL (reference range, 30-100), and PTHrP was normal at 22 pg/mL (reference range, 14-27). Serum calcium reached a nadir of 9.3 mg/dL (reference range, 8.5-10.1). During the course of hospitalization, serum calcium began to rise despite repeated infusions of zolendronic acid. Given inappropriately elevated PTH, hypercalcemia, and normal PTHrP, it was assessed that primary hyperparathyroidism may be the cause of hypercalcemia, and potentially treatable with parathyroid surgery. Endocrine surgery was consulted, and recommended a sestamibi scan, which failed to reveal parathyroid adenomas; however, there was significant uptake over the right upper lung region, left axillary region and bony structures, spatially concordant with the known malignancy. Calcium continued to rise, at which time the health care proxy decided against any further interventions. The patient expired on day 30. Conclusion: PTHrP causes hypercalcemia via similar mechanisms to PTH. In this patient, primary hyperparathyroidism was considered unlikely as the sestamibi scan did not reveal any evidence of parathyroid adenomas; instead, there was increased uptake over the patient’s lung mass, axillary lymph nodes, and metastatic bone lesions. There are known case reports of PTH secreting tumors, which were demonstrated on both sestamibi and immunohistochemistry for intact PTH on tumor tissue. PTH secreting tumors, whilst rare, should be considered in patients with elevated PTH, normal PTHrP levels, and no evidence of primary hyperparathyroidism. Sestamibi scan should be considered as a modality to help diagnose PTH secreting tumors if suspected.