A 25-hydroxy vitamin D test is a test that measures the amount of 25-OH vitamin D in your blood. It's the most accurate way to measure how much vitamin D is in your body. The test can determine whether your vitamin D levels are too high or too low.
The test is also known as the 25-OH vitamin D test and the calcidiol 25-hydroxycholecalciferol test. It can be an important indicator of osteoporosis (bone weakness) and rickets (bone malformation).