ASCP Medical Laboratory Technician (MLT) Practice Exam

The first hydroxylation of vitamin D occurs in the liver. When vitamin D, which is initially obtained from skin synthesis and dietary sources, enters the body, it undergoes a series of biochemical transformations to become biologically active.

In the liver, vitamin D is hydroxylated to form 25-hydroxyvitamin D (calcidiol), which is then released into circulation. This process is crucial because 25-hydroxyvitamin D is the form of vitamin D that is most commonly measured in the blood to assess an individual’s vitamin D status.

The subsequent hydroxylation to its active form, 1,25-dihydroxyvitamin D (calcitriol), takes place primarily in the kidneys. This sequential activation process emphasizes the liver's role as the first site for the initial transformation of vitamin D. Understanding this pathway highlights the importance of liver function in maintaining vitamin D levels and overall health.