Calcitriol

Calcitriol (also called 1,25-dihydroxyvitamin D3), is the active form of D3

• within the kidneys, there exist a number of enzymes which hydroxylate calcidiol (25-hydroxyvitamin D3) at various positions. The most important of these seems to be 1-hydroxylase which produces 1,25-dihydroxycholecalciferol (calcitriol). Calcitriol is more potent than the other hydroxylation products in its actions on mineral physiology. Hence, by varying the activity of the 1-hydroxylase enzyme, the ratio of calcitriol to other hydroxylated products can be varied, and so the degree to which calcium and phosphate physiology is affected
  • renal hydroxylation of calcidiol to calcitriol by 1-alpha-hydroxylase is tightly regulated (stimulated by either parathyroid hormone or hypophosphatemia) and serves as the major control point in production of the most active circulating hormone calcitriol (1,25-dihydroxyvitamin D3, 1,25 - dihydroxycholecalciferol)

1,25-Dihydroxycholecalciferol, calcitriol, is therefore the product of liver and renal hydroxylation of vitamin D3, and is the most active metabolite of vitamin D.

It stimulates calcium uptake by the small intestine and this indirectly promotes mineralization of new bone.

Production of this active form of vitamin D is controlled by parathyroid hormone - PTH - and by serum phosphate concentration: a rise in PTH or a fall in serum phosphate increases 1,25-dihydrocholecalciferol synthesis.

1,25-Dihydroxycholecalciferol also facilitates renal reabsorption of calcium, and increases osteoclastic activity in bone.