8775 The Silent Orchestrator: Tumor Induced Osteomalacia as a Rare Culprit Behind Hypophosphatemia

Abstract Disclosure: M. Montesinos-Cartagena: None. J. Rivera-Rosa: None. C.S. Perez-Draper: None. J. Colon-Castellano: None. Tumor induced osteomalacia (TIO) is a rare disorder characterized by the abnormal metabolism of phosphate and active vitamin D due to overproduction of fibroblast growth factor 23 (FGF23). FGF23 is a hormone that regulates phosphate homeostasis by reducing both phosphate renal reabsorption and production of active vitamin D. Here, we present the case of a 92-year-old male with past medical history pertinent for prostate adenocarcinoma with metastasis to bone on palliative therapy with leuprorelin, enzalutamide, and zoledronic acid who was admitted due to persistent watery diarrheas. Biochemical parameters showed mild hypocalcemia of 8.1 mg/dL (Normal Range (NR), 9-11), hypophosphatemia of 0.8 mg/dl (NR, 2.5- 4.5), albumin of 3.7 g/dl (NR, 3.5-5.5), alkaline phosphatase of 122 IU/l (NR,40-150), preserved renal function with creatinine of 0.79 mg/dl (NR, 0.7-1.2), elevated intact parathyroid hormone (PTH) of 134 pg/ml (NR, 9-52) and active vitamin D deficiency of 11 ng/ml (NR, 30-100 ng/ml). Urine electrolytes remarkable for hyperphosphaturia of 266.8mmol/L. Due to the decreased levels of active vitamin D in this patient with hypophosphatemia, FGF23 levels were measured and came back elevated on 3457 RU/mL (NR, <180RU/mL). Thus, diagnosis of tumor induced osteomalacia was made. The patient was initiated on calcitriol, calcium carbonate and electrolyte replacement was provided. Treatment with burosumab was considered, however due to multiple comorbidities and advanced cancer stage, goals of care were discussed with the patient and his family who opted for hospice care. Our case shows the importance of considering TIO as a possible underlying cause of hypophosphatemia. Moreover, measurement of FGF23 levels is key for diagnosis of TIO, especially in patients with multiple factors contributing to low serum phosphate levels. Adequate diagnosis is essential to determine prognosis and subsequent treatment. Presentation: 6/1/2024