FGF23 and X-linked hypophosphatemia: X-linked hypophosphatemia (XLH) is the most common cause of genetic rickets (1/20 000).1 XLH occurs due to inactivating mutations of Phosphate-regulating Endopeptidase Homolog X-linked (PHEX),2 resulting in increased circulating levels of Fibroblast Growth Factor 23 (FGF23), a bone-derived hormone that leads to renal phosphate wasting and inhibition of the endogenous synthesis of 1,25-(OH)2-vitamin D3.