CYP24A1 and vitamin D deficiency: Other factors involved in CKD-derived vitamin D deficiency include resistance to vitamin D, reduced hepatic synthesis of CYP24A1 [26], and upregulation of 24–25(OH)2D [21], which is a metabolite produced by increased action of the catabolic enzyme CYP24A1, which causes hydroxylation of 25 (OH)D.