Formation of calcitriol: The steroid-like hormone calcitriol is formed from the precursor 7-dehydrocholesterol, which in turn is formed from cholesterol. The hormone passes through several stages in the course of its synthesis: First the skin under the influence of UV light, then the liver and finally the kidney. Calciol (cholecalciferol) is formed in the skin, which is vitamin D3.
The hormone was transported to the liver via the bloodstream and bound to the transport protein vitamin D binding protein. Once it reaches the liver, it is converted to calcidiol and finally to the effective hormone calcitriol in the kidney. The hormone is broken down by an enzyme (24-hydroxylase).
The appropriate receptor of the hormone is located intracellularly. Regulation of Calcitriol: The level of this hormone in the kidney is influenced by the calcium and phosphate concentration in the blood, by parathyroid hormone, by prolactin and by Calcitriol itself. A low calcium level and thus an increased amount of parathormone, a low phosphate concentration and prolactin increase the formation of calcitriol.
The production of the hormone is lowered by calcitriol itself and by larger amounts of calcium and phosphate. The hormone calcitriol affects the intestines, bones, kidneys, placenta, mammary glands and hair. Overall, it promotes calcium absorption via the intestines and the mineralisation of the bones.