Calcifediol

Structure

Calcifediol (INN), alternatively known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D (represented as 25(OH)D), is a significant prehormone generated in the liver. Its synthesis is achieved through the hydroxylation of vitamin D3 (cholecalciferol) by the enzyme named cholecalciferol 25-hydroxylase, an enzyme first isolated by Michael F. Holick. Physicians across the globe utilize this metabolite as a benchmark to gauge a patient's vitamin D status. After its initial formation, calcifediol undergoes further conversion in the kidneys. It can be transformed into calcitriol (1,25-(OH)2D3), which acts as a secosteroid hormone and is identified as the activated form of vitamin D. Another renal conversion results in 24-hydroxycalcidiol through 24-hydroxylation.

Blood test[edit]

In the realm of medical diagnostics, the 25-hydroxy vitamin D (calcidiol) blood assessment serves as a pivotal tool to ascertain the amount of vitamin D present in a patient's body. The concentration of calcidiol in the blood is upheld as the paramount indicator of a patient's vitamin D levels.

This specific assessment is valuable for identifying vitamin D deficiency. It becomes especially crucial for patients exhibiting a higher vulnerability to such a deficiency. Instances include those suffering from conditions like osteoporosis, chronic kidney disorders, malabsorption syndromes, obesity, or certain infections. Despite the prevalence of vitamin D deficiency in populations residing in higher latitudes or those with diminished sun exposure, this test isn't universally recommended. For patients at a lower risk, physicians might suggest over-the-counter vitamin D supplements as an alternative to screening.

While the test is notably sensitive, there has been a call for heightened standardization across labs to ensure reproducibility. According to MedlinePlus, calcidiol's normal concentration range is from 30.0 to 74.0 ng/mL. However, this normal range can deviate considerably, contingent on numerous variables, such as age or geographical factors. Other studies advocate a more expansive reference range (20–150 nmol/L or 8-60 ng/ml), even pinpointing levels below 80 nmol/L (32 ng/ml) as markers of vitamin D deficiency.

It's noteworthy that US laboratories predominantly report 25(OH)D concentrations in ng/mL, whereas other nations may opt for nmol/L. For conversion between these units, multiplying the ng/mL value by 2.5 yields the equivalent in nmol/L.

Clinical significance[edit]

An upward trend in calcifediol concentrations correlates with an enhanced fractional absorption of calcium from the digestive system, a relationship observed up to concentrations of 80 nmol/L (32 ng/mL). It's notable that, regardless of increasing calcifediol levels, urinary calcium excretion (which balances with intestinal calcium absorption) does not surge, even with calcifediol levels peaking at around ~400 nmol/L (160 ng/mL).

In a notable research endeavor, Cedric F. Garland and Frank C. Garland of the University of California, San Diego evaluated blood samples from 25,000 individuals in Washington County, Maryland. Their findings indicated a striking observation: participants with elevated calcifediol concentrations exhibited a colon cancer risk that was merely one-fifth of the standard rates. Nonetheless, subsequent randomized controlled trials did not corroborate a significant association between vitamin D supplementation and reduced colon cancer risks.