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

Figure 3

Autoregulation of calcitriol level. Calcitriol, the active form of vitamin D, is capable of autoregulating its level. When high level of calcitriol is found in the blood, it activates the enzyme CYP24A1, which converts it into excretion product. Low level of calcitriol stimulates production of parathyroid hormone. Parathyroid hormone has two-fold activity: it stimulates CYP27B1 enzyme to produce more of calcitriol from calcidiol and it inhibits the activity of CYP24A1, which does not let calcitriol convert to excretion product.