For example, several studies report elevated serum intact FGF23 (iFGF23) values in the majority of patients with XLH (Yamazaki et al., 2002; Jonsson et al., 2003; Zhang et al., 2019), whereas another study indicates serum iFGF23 values virtually in the reference range (Weber et al., 2003). This evidence concerns the gene FGF23 and X-linked hypophosphatemia.