High iFGF23 level despite hypophosphatemia is one of the clinical indicators to make diagnosis of XLH.

X-linked hypophosphatemic rickets (XLH) is caused by inactivating mutations in the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) gene. Deletion of Phex leads to increased serum fibroblast growth factor23 (FGF23) levels in mouse. The aim is to assure the clinical usefulness of FGF23 determination in the diagnosis of XLH. Participants were 21 patients with XLH having abnormalities in PHEX from 13 kindred (PtPHEX: 1 to 42 years old; 10 males, 11 females) and 55 healthy controls (1 month to 18 years old; 27 males, 28 females). Temporal changes in FGF23 were determined by a single oral phosphate administration in PtPHEX and an ad lib diet in controls. Reference ranges of intact FGF23 (iFGF23) for children were determined. iFGF23 level which distinguish between controls and PtPHEX were validated. Correlations between iFGF23 and the severity of XLH (gender, age of onset, bone deformity, The ratio of maximum rate of renal tubular reabsorption of phosphate to glomerular filtration rate (TmPO(4)/GFR), inorganic phosphate (IP), Alkaline Phosphatase (ALP), therapeutic dose) were investigated. Increasing tendency after phosphate administration and no general tendency after breakfast in iFGF23 were observed. Reference range (5(th) and 95(th) percentiles) of iFGF23 for children (12.9 and 51.2 pg/mL) was similar to that for adults. iFGF23 were above the reference range in 19 of 21 PtPHEX (40 to 4710 pg/mL). iFGF23 did not correlate with any index of severity of XLH. Relatively high iFGF23 despite hypophosphatemia is one of the clinical indicators to diagnose XLH.