While elevation of serum FGF23 reduces the expression of the renal 1α-hydroxylase, hypophosphatemia is normally a stimulator for renal 1α-hydroxylase expression to increase circulating 1,25(OH)2D3[40]; the stimulating effect of hypophosphatemia on the 1α-hydroxylase expression has been well illustrated in the NaPi2a knockout mice (with lower phosphate and lower FGF23 levels in the serum), in which the serum 1,25(OH)2D3 level is elevated due to the increased 1α-hydroxylase expression stimulated by hypophosphatemia [41]. Here, FGF23 is linked to hypophosphatemia.