FGF23 and X-linked hypophosphatemia: X-linked hypophosphatemia (XLH) is a rare, progressive and lifelong disorder caused by pathogenic variants in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, resulting in excess fibroblast growth factor 23 (FGF23) and consequent renal phosphate wasting (1–3) affecting approximately 1 in 20,000 to 70,000 people globally (4–7).