FGF23 and hypophosphatemia: In a subsequent study, however, FGF23 did not reduce serum phosphorus levels in double Fgfr3/Fgfr4 KO mice [18], suggesting that FGFR3 and FGFR4 play redundant roles in phosphate regulation, and perhaps that FGFR1 activation is not sufficient for FGF23 to induce hypophosphatemia.