Cholecalciferol (vitamin D₃) is formed in the skin following exposure to ultraviolet light and it is taken in through food (e.g. fatty fish, dairy products). The vitamin is tranformed into active 1.25-dihydroxycholecalciferol (1.25(OH)2D3, calcitriol) in two hydroxylation steps, first in the liver,then in the kidneys. Calcitriol is a highly efficient steroid hormone. It regulates the metabolism of calcium and phospahtes by stimulating their intestinal absorption, the mobilisation from the bones, and the renal reabsorption of calcium. Through another specific receptor calcitriol also modulates many other (partially unexplained) cellular procedures in the entire body.

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