Tumor-induced osteomalacia with normal systemic fibroblast growth factor-23 level

A 38-year-old man presenting with long bone/rib fractures was diagnosed with tumor-induced osteomalacia (TIO) caused by a giant cell tumor in the right foot with normal systemic fibroblast growth factor-23 (FGF23) levels. Multiple imaging modalities done initially and one year later were unable to localize the tumor. New-onset foot pain discovered a right foot mass with resolution of metabolic abnormalities post-surgery. Sampling from both femoral veins showed an elevated FGF23 value on the right side. This case is unique in that the patient had a normal systemic FGF23 level even with severe clinical manifestations of TIO.