PTH and chronic kidney disease: Multiple regression analysis of 1,25VitD3 using eGFR and serum FGF23, intact PTH and calcium as explanatory factors across CKD patients at stages 1, 2 and 3 revealed 1,25VitD3 levels to be significantly and inversely correlated with FGF23 (β = −0.493, P<0.0001) and positively correlated with eGFR (β = 0.213, P = 0.0028) as independent contributing factors (R2 = 0.310, P<0.0001) (Table S2).