Circulating FGF23 is influenced by serum calcium and VDR levels. Basal serum biochemical parameters of WT, VDR−/−, and 1(OH)ase−/− mice. Serum Ca, P, PTH, and FGF23 are shown in panels (A–D), respectively. WT mice were maintained on normal chow. 1(OH)ase−/− and VDR−/− mice were maintained either on a high-Ca diet or a rescue diet, denoted as 1(OH)ase−/−C and VDR−/−C, and 1(OH)ase−/−R and VDR−/−R, respectively. Bars represent the mean±SEM of measured data from six to eight animals of each genotype. ♦♦♦P < 0.001 compared with age-matched WT; ***P<0.001 and **P≤0.01 compared with mice of the same genotype on a high-Ca diet.