Malignancy-mimicking osteitis fibrosa cystica secondary to primary hyperparathyroidism.

Osteitis fibrosa cystica (OFC) is a rare skeletal complication of primary hyperparathyroidism (PHPT) that may present with multifocal osteolytic lesions mimicking multiple myeloma (MM) or metastatic disease. We report a 79-year-old woman with several years of generalized bone pain, fatigue, anorexia, and weight loss. Imaging revealed disseminated lytic lesions throughout the axial and appendicular skeleton, and MM was initially suspected. However, serum protein electrophoresis was normal, and bone marrow biopsy showed reactive changes with polyclonal plasma cells. She later developed a pathological midshaft femoral fracture. Laboratory testing showed severe hypercalcemia, markedly elevated parathyroid hormone, hypophosphatemia, elevated alkaline phosphatase, and vitamin D deficiency. Fracture fixation with targeted bone biopsy confirmed brown tumor consistent with OFC, and sestamibi scintigraphy localized a right inferior parathyroid adenoma. This case highlights that PHPT should be considered in patients with multifocal lytic bone lesions.