Calcifediol vs. Calcitriol

When testing vitamin D levels in the body, healthcare professionals typically measure the amount of 25-hydroxyvitamin D, also known as calcifediol, rather than 1,25-dihydroxyvitamin D or calcitriol, the active form of the vitamin. This is due to several reasons:

-Higher concentration: Calcifediol is present in the blood in higher concentrations than calcitriol.

-Longer half-life: Calcifediol has a longer half-life in the body than calcitriol (approximately 2 weeks compared to 4 to 6 hours for calcitriol). Therefore, calcifediol levels provide a more stable and accurate reflection of vitamin D status over time.

-Regulatory mechanisms: The kidneys tightly control the conversion of calcifediol to calcitriol as part of the body’s regulation of calcium and phosphorus. Even if a person is deficient in vitamin D, their calcitriol levels may be normal or even elevated due to these regulatory mechanisms. Therefore, calcitriol levels are not a reliable indicator of total vitamin D status in the body.

It is important to note that although measuring calcifediol is the standard method of assessing vitamin D status, it does not provide a measure of all aspects of vitamin D health, such as its conversion to its active form or its effects on cell signaling and gene expression. Therefore, a comprehensive assessment of a person’s health and other testing may be necessary to fully understand a person’s health status.