[Vitamin D](vitamin-d), also known as calcitriol in its active form, is produced in the kidneys and helps the body absorb calcium, among other important functions like supporting the immune system. It is made from another form of vitamin D (/vitamins/vitamin-d/calcifediol) and is carefully regulated to keep its levels in check. While calcitriol is the active form, it’s not a good indicator of overall vitamin D storage in the body. Instead, vitamin D levels in storage are better measured by checking calcifediol levels. Some diseases, like sarcoidosis, can cause too much active vitamin D, while others, like chronic kidney disease, can prevent enough from being made.