Vitamin D, Calcium, Parathyroid Hormone, and Sex Steroids in Bone Health and Effects of Aging

Figure 4

Effect of calcitriol on serum calcium level. Calcitriol, the active form of vitamin D, causes vitamin D receptor (VDR) to dimerize with retinoid X receptor. This causes activation of vitamin D response element in the genome. In the intestine, this further results in increased absorption of calcium from food and leads to increased serum calcium level. When the serum calcium level drops, calcitriol and PTH are activated. This causes increased calcium absorption from the kidneys and bone, where there is largest deposit of calcium. This absorption again leads to increased serum calcium.