FGF23 and X-linked hypophosphatemic rickets: Inappropriately high FGF23 signaling and resulting chronic hypophosphatemia are associated with disorders of FGF23 excess such as tumor-Induced osteomalacia (TIO) in adults and X-linked hypophosphatemic rickets (XLH) in children (5, 6), associated with fatigue, muscle weakness, fractures, and pain.