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SUN-748 The Importance of Baseline Phosphorus Testing in Bone Disorders. A Case Report

G. Loria, Natalia Fretes Oviedo, Xiangling Wang, Ilaslan Hakan, Mary-Beth Roberts, Elizabeth File, L. Khan

Journal of the Endocrine Society, 2025

Abstract

Abstract Disclosure: G. Madrigal Loria: None. N. Fretes Oviedo: None. X. Wang: None. I. Hakan: None. M. Roberts: None. E. File: None. L. Khan: None. Background: Serum phosphorus is a critical yet often overlooked diagnostic marker in bone disorders. Routine assessment is essential in patients with bone pain, rib fractures, or skeletal abnormalities, as delayed recognition of hypophosphatemia can impact diagnosis and treatment. Elevated FGF23 suggests tumor-induced osteomalacia (TIO) or genetic disorders such as X-linked hypophosphatemia (XLH). Phosphaturic mesenchymal tumors (PMTs) are the most common cause of TIO, but other malignancies can also induce phosphate wasting. If FGF23 is normal, other causes, such as vitamin D deficiency or malabsorption, should be considered. Case Presentation:A 36-year-old male presented with progressive bone pain, and mobility impairment over two years. Initial workup did not include phosphorus testing, delaying diagnosis. Imaging revealed chronic skeletal fractures, and prior treatment with zoledronic acid worsened symptoms. His history included premature puberty, but no known familial bone disease. Physical exam showed skeletal tenderness and difficulty ambulating. Labs revealed severe hypophosphatemia (1.3 mg/dL), elevated FGF23 (191 pg/mL), increased urinary phosphate excretion, secondary hyperparathyroidism, and elevated alkaline phosphatase. Imaging showed diffuse skeletal radiotracer uptake but no FDG-avid lesions. Multiple myeloma and…

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