SAT-794 - The Forgotten Electrolyte: A Three‑Year Diagnostic Odyssey of Tumor‑Induced Osteomalacia

Introduction:
Tumor induced osteomalacia (TIO) is a rare paraneoplastic disorder caused by fibroblast growth factor 23 (FGF23) mediated renal phosphate wasting, leading to hypophosphatemia, osteomalacia, bone pain, and muscle weakness. Due to its rarity and nonspecific presentation, diagnosis is frequently delayed, often after evaluation by multiple specialties. We present a case highlighting the diagnostic challenges of TIO, the critical importance of serum phosphate in recognizing this condition, and the role of multidisciplinary collaboration in achieving definitive care.

Case Presentation:
A 38-year-old woman presented with progressive bilateral hip pain and lower extremity weakness beginning in 2022, without trauma. Her symptoms worsened over three years, ultimately necessitating the use of a rolling walker for ambulation. She was evaluated by multiple specialties, including orthopedics, physical medicine and rehabilitation, neurology, and rheumatology, with initial diagnoses of greater trochanteric bursitis, piriformis syndrome, and possible neurologic etiologies. Multiple spine and brain MRIs were unrevealing, and initial hip radiographs were normal. Despite physical therapy and corticosteroid injections, her symptoms progressed. Subsequent orthopedic reassessment revealed proximal muscle weakness, and hip MRI demonstrated acute bilateral femoral neck stress fractures, prompting evaluation for metabolic bone disease. Laboratory studies showed significant hypophosphatemia (serum phosphorus 1.6 mg/dL [2.5–4.5]) with markedly elevated bone‑specific alkaline phosphatase 65.4 mcg/L (5.3–19.5). Endocrinology evaluation demonstrated renal phosphate wasting (24‑hour urine phosphorus 616 mg). FGF23 was inappropriately normal at 93 RU/mL (44–215) in the setting of hypophosphatemia, and 1,25‑dihydroxyvitamin D was also inappropriately normal at 44 pg/mL (18–72). These findings were consistent with FGF23‑mediated hypophosphatemia. With a tentative diagnosis of tumor-induced osteomalacia, treatment with calcitriol and phosphate supplementation was initiated, leading to improvement in strength and pain. Functional imaging with FDG PET/CT localized an FDG-avid lesion in the distal left femur, suspicious for a phosphaturic mesenchymal tumor. Further MRI characterization and multidisciplinary surgical planning are ongoing.

Conclusion:
This case illustrates the prolonged diagnostic delay commonly associated with tumor induced osteomalacia and underscores the importance of measuring serum phosphate in patients with unexplained bone pain, muscle weakness, or stress fractures. Early recognition of biochemical abnormalities and coordinated multidisciplinary evaluation are essential to localize the causative tumor and pursue curative resection, preventing years of morbidity from this potentially reversible condition.

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