SUN-820 - A Malignant-Appearing Parathyroid Neoplasm Presenting as Normocalcemic Primary Hyperparathyroidism with Progressive Osteoporosis

Introduction
Normocalcemic primary hyperparathyroidism (NPHPT) is characterized by elevated parathyroid hormone (PTH) levels in the setting of normal serum calcium and is frequently managed conservatively. Although most cases follow a benign course, the potential for progression to aggressive parathyroid disease remains incompletely understood.
Case Presentation
A 61-year-old woman with hypertension and osteoporosis was referred for evaluation of persistently elevated PTH. Initial laboratory testing demonstrated PTH of 140 pg/mL with normal total calcium (10.0 mg/dL), normal albumin (4.0 g/dL), and sufficient vitamin D levels (65 ng/mL). Subsequent endocrinology evaluation confirmed NPHPT, with persistently elevated PTH (104 pg/mL), normal total calcium, mildly elevated ionized calcium (5.5 mg/dL), and exclusion of secondary causes. Given biochemical stability, absence of hypercalcemic symptoms, and mild osteoporosis, close monitoring was initially pursued. Despite bisphosphonate therapy, bone mineral density progressively worsened. Repeat evaluation demonstrated rising PTH levels (140 pg/mL) with new mild hypercalcemia (total calcium 10.8 mg/dL, ionized calcium 5.6 mg/dL) and elevated 24-hour urine calcium (254 mg). Parathyroid-protocol computed tomography revealed a 9 cm heterogeneously enhancing mass posterior to the lower pole of the left thyroid lobe extending into the superior mediastinum, with moderate tracheal deviation and imaging features concerning for an aggressive parathyroid neoplasm. Surgical resection is planned, and histopathologic evaluation is pending.
Discussion
This case illustrates the evolving biochemical and clinical spectrum of NPHPT and emphasizes the importance of longitudinal monitoring. Although initially normocalcemic and asymptomatic, the patient developed progressive skeletal disease refractory to medical therapy and later biochemical progression to overt hyperparathyroidism. The presence of a large parathyroid mass with concerning imaging features raises suspicion for atypical adenoma or parathyroid carcinoma. NPHPT may represent an early manifestation of more severe parathyroid disease, and progression of bone loss or structural abnormalities should prompt timely imaging and surgical evaluation.
Conclusion
In patients with NPHPT, worsening skeletal disease or rising biochemical markers should not be dismissed, even in the absence of longstanding hypercalcemia, as early recognition may prevent diagnostic delay in high-risk parathyroid disease.

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