Calcifediol also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D (abbreviated 25(OH)D), is a prehormone

Prehormone

A prehormone is a biochemical substance secreted by glandular tissue and has minimal or no significant biological activity, but it is converted in peripheral tissues into an active hormone....

 that is produced in the liver

Liver

The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 by hydroxylation

Hydroxylation

Hydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...

 of vitamin D₃ (cholecalciferol) by the enzyme cholecalciferol 25-hydroxylase. Calcifediol is then converted in the kidneys (by the enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 25(OH)D-1α-hydroxylase

25-Hydroxyvitamin D3 1-alpha-hydroxylase

25-Hydroxyvitamin D3 1-alpha-hydroxylase also known as cytochrome p450 27B1 is a cytochrome P450 enzyme that in humans is encoded by the CYP27B1 gene....

) into calcitriol

Calcitriol

Calcitriol , also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3, is the hormonally active form of vitamin D with three hydroxyl groups...

 (1,25-(OH)₂D₃), a secosteroid

Secosteroid

A secosteroid is a molecule similar to a steroid but with a "broken" ring. Etym: L. Seco, to cut, _ steroidSecosteroids are very similar in structure to steroids except that two of the B-ring carbon atoms of the typical four steroid rings are not joined, whereas in steroids they are.In humans, the...

 hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 that is the active form of vitamin D. It can also be converted into 24-hydroxycalcidiol in the kidneys via 24-hydroxylation.

Blood test

In medicine, a 25-hydroxy vitamin D (calcidiol) blood test is used to determine how much vitamin D is in the body. The blood concentration of calcidiol is considered the best indicator of vitamin D status. It is the most sensitive

Sensitivity and specificity

Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as classification function. Sensitivity measures the proportion of actual positives which are correctly identified as such Sensitivity and specificity are statistical...

 measure, though experts have called for improved standardization and reproducibility across different laboratories. According to MedlinePlus

MedlinePlus

MedlinePlus is a free Web site that provides consumer health information for patients, families, and Health care providers. The site brings together information from the United States National Library of Medicine, the National Institutes of Health , other U.S. government agencies, and...

, the normal range of calcidiol is 30.0 to 74.0 ng/mL. The normal range varies widely depending on several factors, including age and geographic location. A broad reference range of 20–150 nmol/L (8-60 ng/ml) has also been suggested, while other studies have defined levels below 80 nmol/L (32 ng/ml) as indicative of vitamin D deficiency.

Clinical significance

Increasing calcidiol levels are associated with increasing fractional absorption of calcium from the gut up to levels of 80 nmol/L (32 ng/mL). Urinary calcium excretion balances intestinal calcium absorption and does not increase with calcidiol levels up to ~400 nmol/L (160 ng/mL).

A study by Cedric F. Garland and Frank C. Garland

Frank C. Garland

Frank Caldwell Garland was an American epidemiologist whose research led to the conclusion that vitamin D deficiency can be a factor increasing risk for breast cancer and colon cancer.-Biography:...

 of the University of California, San Diego

University of California, San Diego

The University of California, San Diego, commonly known as UCSD or UC San Diego, is a public research university located in the La Jolla neighborhood of San Diego, California, United States...

 analyzed the blood from 25,000 volunteers from Washington County, Maryland

Washington County, Maryland

Washington County is a county located in the western part of the U.S. state of Maryland, bordering southern Pennsylvania to the north, northern Virginia to the south, and the Eastern Panhandle of West Virginia to the south and west. As of the 2010 Census, its population is 147,430...

, finding that those with the highest levels of calcifediol had a risk of colon cancer that was one-fifth of typical rates.

See also

• Hypervitaminosis D
  Hypervitaminosis D
  Hypervitaminosis D is a state of vitamin D toxicity.The recommended daily allowance is 400 IU per day. Overdose has been observed at 1925 µg/d...
  
  
• Hypovitaminosis D
  Hypovitaminosis D
  Hypovitaminosis D is a deficiency of Vitamin D. It can result from: inadequate nutritional intake of vitamin D coupled with inadequate sunlight exposure , disorders that limit vitamin D absorption, and conditions that impair the conversion of vitamin D into active metabolites including certain...
  
  
• Vitamin D
  Vitamin D
  Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
  
  
  • Health effects