25-hydroxyvitamin D
25-hydroxyvitamin D is the body's vitamin D savings account. It is the half-activated, stored form that floats in the blood waiting to be turned into the fully active hormone when needed. Because it reflects how much vitamin D you have banked over weeks, it is the number doctors check to judge your vitamin D status.
It is made in the liver, which adds the first of two activating chemical groups to vitamin D. It then circulates bound to a carrier protein and has a relatively long half-life, which is exactly why it is a reliable marker of overall supply rather than minute-to-minute demand. The kidney later converts it to calcitriol, the active form, on demand.
When this storage level is low, the body has too little raw material, and PTH often rises to compensate, an early warning sign. Optimal thresholds are debated, but very low levels clearly cause bone disease, while very high levels from over-supplementation can be toxic. Measuring calcidiol, not the active calcitriol, is the standard way to detect deficiency.
An office worker who avoids the sun has a low 25-hydroxyvitamin D level on routine testing, prompting a supplement to rebuild the body's stores.
This storage form is the standard screening test for vitamin D status.
Confusingly, the stored form (25-hydroxyvitamin D) is what is measured, while the active form (calcitriol) is what actually works. The two should not be confused on a lab report.