Severe Tertiary Hyperparathyroidism with Brown Tumor and Multisystem Complications Mimicking Plasma Cell Dyscrasia in a Middle-Aged Man with Childhood-Onset FSGS.

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is a well-recognized complication of end-stage kidney disease (ESKD), encompassing abnormalities in calcium, phosphorus, parathyroid hormone (PTH), vitamin D metabolism, and bone turnover. In advanced cases, secondary or tertiary hyperparathyroidism may lead to severe skeletal disease, including osteitis fibrosa cystica and brown tumors, which can mimic hematologic malignancies. We present the case of a 47-year-old man with childhood-onset focal segmental glomerulosclerosis (FSGS), two prior kidney transplants, and long-standing dialysis dependence who presented with bone pain, pancytopenia, neurologic symptoms, and imaging abnormalities initially concerning for multiple myeloma. Comprehensive evaluation revealed a single unifying diagnosis: severe tertiary hyperparathyroidism (parathyroid hormone 1,467 pg/mL) with advanced renal osteodystrophy and brown tumor-like skeletal changes, without evidence of plasma cell dyscrasia or marrow infiltration. This case highlights the diagnostic pitfalls posed by extreme manifestations of CKD-MBD and underscores the importance of prioritizing cohesive pathophysiologic reasoning in complex ESKD patients.