On the other hand, univariate analyses showed that serum FGF23 levels significantly and inversely correlate with serum 1,25VitD3 levels in CKD patients at stages 1 (r = −0.542, P<0.0001), 2 (r = −0.533, P<0.0001), 3(r = −0.344, P = 0.0112) and 4–5 (r = −0.525, P = 0.0006) (Fig. 4A–D). Here, FGF23 is linked to chronic kidney disease.