FGF23 and hypophosphatemia: XLH is a rare congenital bone condition caused by inactivating mutations in the PHEX gene, which leads to upregulation of FGF23 from the bone compartment and resultant hypophosphatemia.29 Sporadic cases appear to represent about 20% to 30% of cases.30 XLH is often mistaken for the more common nutritional rickets, with children with XLH showing increased serum alkaline phosphatase activity as well as lower‐extremity bowing, rachitic features, and/or metaphyseal dysplasia.