[Vitamin D](vitamin-d), also called calcifediol (the short-term storage form), is made in the liver from another form of vitamin D. It usually binds to a protein in the blood, which helps it stay in the body for weeks. If there's not enough of this protein, vitamin D can be broken down and removed from the body through the kidneys. When the body needs more of the active form of vitamin D (/vitamins/vitamin-d/calcitriol), especially for things like low calcium or immune system support, stored vitamin D is quickly converted. [Vitamin D](vitamin-d) levels in the blood can indicate if someone has a deficiency or not. If levels are too low (see table below), the body may struggle to make enough active vitamin D, leading to various diseases.