The conversion of calcifediol to calcitriol is catalyzed by the enzyme 25-hydroxyvitamin D 3 1-alpha-hydroxylase, the levels of which are increased by parathyroid hormone (and additionally by low calcium or phosphate).
The conversion of calcifediol to calcitriol is catalyzed by the enzyme 25-hydroxyvitamin D 3 1-alpha-hydroxylase, which is the product of the "CYP27B1" human gene. The activity of CYP27B1 is increased by parathyroid hormone (and additionally by low calcium or phosphate).