Calcitriol

Calcitriol is the principal hormonal form of vitamin D, chemically known as 1,25-dihydroxyvitamin D. It is produced after vitamin D undergoes two hydroxylation steps. Vitamin D is first converted mainly in the liver to 25-hydroxyvitamin D, or calcidiol. Calcidiol is then converted primarily in the kidney by the enzyme CYP27B1, also called 1-alpha-hydroxylase, to calcitriol .^1,2

Renal calcitriol production is tightly regulated as part of calcium and phosphate homeostasis. Parathyroid hormone stimulates CYP27B1 activity, whereas fibroblast growth factor 23 and calcium and phosphate status contribute to its regulation. Some tissues outside the kidney can also produce calcitriol, where it may act locally rather than substantially contributing to circulating hormone concentrations .¹

Calcitriol acts mainly by binding the vitamin D receptor, a nuclear receptor that regulates gene transcription. One of its major physiological effects is to increase intestinal absorption of calcium and phosphate. Through its actions on mineral metabolism, including effects involving the intestine, kidneys, bone, and parathyroid glands, calcitriol contributes to calcium and phosphate homeostasis and skeletal mineralization .^1,2

Calcitriol is distinct from vitamin D itself and from calcidiol. Vitamin D obtained from skin synthesis or diet is a precursor, while calcidiol is the major circulating form and is generally used to assess vitamin D status. Calcitriol circulates at much lower concentrations, is more tightly hormonally regulated, and represents the principal active endocrine form of the vitamin D system .^1,2