FGF23 and hypophosphatemia: XLHR is a dominantly inherited disease caused by mutations of the PHEX gene, with a prevalence of 1 in 20000 individuals.[1] It is characterized by hypophosphatemia, aberrant vitamin D metabolism, growth retardation and bone mineralization defects, and is associated with a pathological elevation of fibroblast growth factor 23 (FGF23).