FGF23 and X-linked hypophosphatemia: X-linked hypophosphatemia (XLH) is a rare musculoskeletal disease caused by loss-of-function mutations in the PHEX gene, resulting in excess production of fibroblast growth factor 23 (FGF23) [1], [2]. The excess plasma FGF23 disrupts renal phosphate reabsorption and 1,25-dihydroxyvitamin D, resulting in hypophosphatemia and defective bone mineralization, followed by rickets, osteomalacia, short stature, bone and joint pain, skeletal abnormalities, and reduced physical function [3], [4], [5].