Calcitriol

Calcitriol is the active form of vitamin D3 (cholecalciferol). Chemically, it is a steroid hormone that plays a role in calcium metabolism and its regulation in the human body. Vitamin D3 is synthesised in the skin (stratum basale) when exposed to the UV light or is ingested in food (e.g. dairy, cereals, fish). Vitamin D is the ONLY vitamin that the human body can produce on its own.

The active metabolite calcitriol is formed in the proximal convoluted tubules of the kidneys by the way of hydroxylation of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol (enzyme 1α-hydroxylase).

Biological function

 * Increased resorption of calcium and phosphate in the small intestine
 * Stimulation of bone mineralisation & remodeling at low/normal vitamin D levels
 * Stimulation of bone resorption at high vitamin D levels – leads to increased serum calcium and phosphate levels
 * Together with PTH acts on the kidney and increases resorption of calcium, calcitriol also increases reabsorption of phosphates in the kidney (PTH stimulates phosphate excretion)
 * Regulated by negative-feedback loop: hypercalcemia leads to decreased activity of both calcitriol and PTH

Transport & Storage
Vitamin D and its metabolites are transported in blood to target cells bound to a vitamin D-binding protein (DBP) which is a glycoprotein. Only 1% of vitamin D is free in blood. The biological half life is only a couple of hours, but as a steroid hormone, vitamin D can be stored in the adipose tissue as 25-hydroxycholecalciferol.

Related articles

 * Vitamin D
 * Ergocalciferol
 * Calcium-phosphate metabolism
 * Osteomalacia