FGF23 and vitamin D deficiency: Conversely, iron deficiency may also cause vitamin D deficiency by decreasing the activity of heme-containing vitamin D-activating enzymes such as 25- and 1α-hydroxylase, as demonstrated in rats [7], or by increasing fibroblast growth factor 23 (FGF23) [8], which suppresses the 1α-hydroxylation of vitamin D [9].