Calcitriol molecule. Computer model showing the structure of a molecule of the secosteroid hormone calcitriol. Atoms are colour-coded (carbon: grey, hydrogen: white, oxygen: red) with the bonds between them shown as sticks. Calcitriol is an active form of vitamin D (cholecalciferol) produced from calcifediol by enzymes in the kidneys. Calcifediol itself is produced by the hydroxylation of vitamin D3 in the liver. Calcitriol increases gastrointestinal calcium absorption, stimulates osteoclastic calcium resorption from bone, facilitates the effect of parathyroid hormone (PTH) on bone resorption, and increases renal (kidney) tubular absorption of calcium.
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