Tumor-induced osteomalacia caused by fibroblast growth factor 23 excess : Delayed diagnosis resulting from overlooked hypophosphatemia-A  case report.

Tumor-induced osteomalacia is a rare acquired disorder of phosphate metabolism caused by fibroblast growth factor (FGF)23-secreting mesenchymal tumors. We report the case of a 39-year-old man with a 3-year history of progressive musculoskeletal pain, in whom unrecognized hypophosphatemia led to misdiagnoses, including osteoporosis and inflammatory musculoskeletal disorders, and consequently, to inappropriate management. Biochemical evaluation revealed persistent hypophosphatemia, elevated alkaline phosphatase levels, secondary hyperparathyroidism, and markedly increased serum FGF23 levels. Gallium-68 DOTATATE positron emission tomography/computed tomography (PET/CT) identified a phosphaturic mesenchymal tumor located in the medial condyle of the right distal femur, which was subsequently confirmed histopathologically after surgical excision. Following surgery, serum phosphorus and FGF23 levels rapidly normalized, accompanied by significant clinical improvement. This case highlights unrecognized hypophosphatemia as a critical yet preventable cause of diagnostic delay and emphasizes the importance of routine serum phosphate assessment to avoid misdiagnosis and enable timely curative treatment.