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The name 25(OH)D refers to any combination of calcifediol (25-hydroxy-cholecalciferol), derived from vitamin D 3, and ercalcidiol (25-hydroxy-ergocalciferol), [1] derived from vitamin D 2. The first of these (also known as 25-hydroxy vitamin D3) is made by the body, or is sourced from certain animal foods or cholecalciferol supplements.
A third important effect of PTH on the kidneys is stimulation of the conversion of 25-hydroxy vitamin D into 1,25-dihydroxy vitamin D (calcitriol). [216] This form of vitamin D is the active hormone which promotes calcium uptake from the intestine via the action of calbindin. [218] Calcitriol also reduces calcium loss to urine. [215]
Most vitamin D is converted to 25(OH)D in the serum, giving an accurate picture of vitamin D status. [53] The level of serum 1,25(OH)D (calcitriol) is not usually used to determine vitamin D status because it often is regulated by other hormones in the body such as parathyroid hormone. [53] The levels of 1,25(OH)D can remain normal even when a ...
It requires two hydroxylations to become active: the first in the liver by CYP2R1 to form 25-hydroxyergocalciferol (ercalcidiol or 25-OH D 2 [20]), and the second in the kidney by CYP27B1, to form the active 1,25-dihydroxyergocalciferol (ercalcitriol or 1,25-(OH) 2 D 2), which activates the vitamin D receptor. [21]
Alfacalcidol (or 1-hydroxycholecalciferol) is an analogue of vitamin D used for supplementation in humans and as a poultry feed additive. Alfacalcidol has a weaker impact on calcium metabolism [ 3 ] and parathyroid hormone levels [ 4 ] than calcitriol ; but significant effects on the immune system , including regulatory T cells . [ 5 ]
Vitamin D toxicity, or hypervitaminosis D, is the toxic state of an excess of vitamin D.The normal range for blood concentration of 25-hydroxyvitamin D in adults is 20 to 50 nanograms per milliliter (ng/mL).
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Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [8] [9] [10] It is also known as 1,25-dihydroxycholecalciferol.It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [11]