The 25-OH Vitamin D test measures the amount of vitamin D in your body. It can be used to check if you have enough vitamin D, which is important for healthy bones and muscles. It can also be used to monitor the effectiveness of any vitamin D, calcium, magnesium, or phosphorus supplements you may be taking. This test is especially important for people with diseases that interfere with fat absorption, such as cystic fibrosis or Crohn's disease.