Vitamin D (25-OH)
What it measures, and what changes the number.
This test measures 25-hydroxyvitamin D (calcidiol), the main circulating storage form of vitamin D. The body makes vitamin D in the skin with sunlight and absorbs it from food, then the liver converts it to this form, which is the standard way of assessing vitamin D status.
It is often read alongside calcium, phosphate, PTH, alkaline phosphatase and magnesium, because vitamin D, PTH and these minerals regulate one another in bone and mineral balance.
Levels vary with season and sun exposure, so the time of year matters when comparing results. Methods differ: immunoassays (the IA in some report names) and mass spectrometry can give different values, and some methods count D2 and D3 forms differently. Units also vary between labs. Labs may print it as 25 Hidroxivitamina D (calcidiol), VITAMIN D,25-OH,TOTAL,IA, Vitamin D, or Vitamina D 25OH (calcidiol).
Often read alongside: Calcium (total), PTH (intact), Phosphate, ALP, Magnesium