FGF23 and X-linked hypophosphatemia: XLH is caused by pathologic loss-of-function mutations in the PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) gene, located on Xp22.1, which causes an elevation of circulating levels of fibroblast growth factor 23 (FGF23), which regulates renal phosphate reabsorption and the production of calcitriol (4).