FGF23 and X-linked hypophosphatemia: The following laboratory tests are indicated for the diagnostic evaluation of the XLH patient [6–8, 14]: serum phosphate, serum calcium, bone-specific alkaline phosphatase (ALP), parathyroid hormone (PTH), 25(OH) vitamin D, 1,25(OH)2 vitamin D, FGF23, serum creatinine, urinary calcium: creatinine ratio, urinary phosphate and creatinine levels to be used for the calculation of TmPO4/GFR.