FGF23 and X-linked hypophosphatemia: Loss of PHEX results in elevated fibroblast growth factor 23 (FGF23), a circulating phosphaturic hormone that acts on the kidney to decrease phosphate reabsorption and inhibit 1,25-dihydroxyvitamin D production.1 The resulting hypophosphatemia contributes to poor mineralization of the extracellular matrix in the bone and teeth of XLH patients.