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

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 metabolic disorders were ruled out and genetic testing, including PHEX, was negative. Treatment with calcitriol (0.25 mcg/d) and phosphorus (250 mg four times daily) did not lead to symptomatic improvement but dosing is still getting titrated and whole-body PET/CT is pending.Worsening Symptoms with Zoledronic Acid:Zoledronic acid exacerbated symptoms by further suppressing bone turnover, worsening osteomalacia, fractures, and bone pain. This highlights the risks of bisphosphonates in phosphate-wasting disorders.Clinical Implications:With hypophosphatemia, elevated FGF23, and no family history, TIO was the leading diagnosis, though XLH remained a possibility. TIO can be cured by tumor removal, while XLH requires lifelong management. Various tumors, including PMTs, sarcomas, and metastatic carcinomas, can lead to acquired phosphate-wasting syndromes, emphasizing the importance of thorough evaluation. Conclusion: Serum phosphorus should be routinely assessed in patients with unexplained bone disease to differentiate between genetic and acquired phosphate-wasting disorders. Presentation: Sunday, July 13, 2025