US labs generally report 25(OH)D levels as ng/mL. This measurement is considered the best indicator of overall vitamin D status. Clinical tests for 25(OH)D often measure the total level of both of these two compounds together, generally without differentiating. The second (25-hydroxy vitamin D2) is from certain vegetable foods or ergocalciferol supplements. 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. The name 25(OH)D refers to any combination of calcifediol (25-hydroxy-cholecalciferol), derived from vitamin D 3, and ercalcidiol (25-hydroxy-ergocalciferol), derived from vitamin D 2. In medical practice, a blood test for 25-hydroxy-vitamin D, 25(OH)D, is used to determine an individual's vitamin D status. Main article: Vitamin D § Recommended serum levels

This will inactivate calcitriol to calcitroic acid, but 24,25-(OH) 2D 3 may have some biological actions itself. This enzymatic reaction is performed by CYP24A1 which is expressed in many vitamin D target tissues including kidney, and is induced by calcitriol. Ĭalcifediol is also converted into 24,25-dihydroxycholecalciferol by 24-hydroxylation. CYP27B1 is also expressed in a number of other tissues, including macrophages, monocytes, keratinocytes, placenta and parathyroid gland and extra-renal synthesis of calcitriol from calcifediol has been shown to have biological effects in these tissues. This enzymatic 25(OH)D-1α-hydroxylase reaction is performed exclusively by CYP27B1, which is highly expressed in the kidneys where it is principally regulated by parathyroid hormone, but also by FGF23 and calcitriol itself. Ĭalcifediol is further hydroxylated at the 1-alpha-position in the kidneys to form 1,25-(OH) 2D 3, calcitriol. Calcifediol has an elimination half-life of around 15 to 30 days. Calcifediol binds in the blood to vitamin D-binding protein (also known as gc-globulin) and is the main circulating vitamin D metabolite. When large doses are given (100,000 IU), it takes 7 days to reach peak calcifediol concentrations. Īt a typical intake of cholecalciferol (up to 2000 IU/day), conversion to calcifediol is rapid. Similarly, vitamin D 2, ergocalciferol, can also be 25-hydroxylated to form 25-hydroxyergocalciferol, (ercalcidiol, 25(OH)D 2) both forms are measured together in blood as 25(OH)D. Variations in the expression and activity of CYP2R1, such as low levels in obesity, affect circulating calcifediol. This enzymatic 25-hydroxylase reaction is mostly due to the actions of CYP2R1, present in microsomes, although other enzymes such as mitochondrial CYP27A1 can contribute.

It is synthesized in the liver, by hydroxylation of cholecalciferol (vitamin D 3) at the 25-position. Biology Ĭalcifediol is the precursor for calcitriol, the active form of vitamin D. Calcifediol is available as an oral medication in some countries to supplement vitamin D status. Measurement of serum calcifediol is the usual test performed to determine a person's vitamin D status, to show vitamin D deficiency or sufficiency. Ĭalcifediol is strongly bound in blood by the vitamin D-binding protein. Calcifediol can be further hydroxylated by the enzyme 25(OH)D-1α-hydroxylase, primarily in the kidney, to form calcitriol (1,25-(OH) 2D 3), which is the active hormonal form of vitamin D. Calcifediol, also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D 3 (abbreviated 25(OH)D 3), is a form of vitamin D produced in the liver by hydroxylation of vitamin D 3 (cholecalciferol) by the enzyme vitamin D 25-hydroxylase.

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