Vitamin D is a family of compounds that is essential for the proper growth and formation of teeth and bones. This test measures the level of vitamin D in the blood.The main role of vitamin D is to help regulate blood levels of calcium, phosphorus, and (to a lesser extent) magnesium. Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in diseases called rickets in children and osteomalacia in adults. Vitamin D has also been shown to influence the growth and differentiation of many other tissues and to help regulate the immune system. These other functions have implicated vitamin D in other disorders, such as autoimmunity and cancer.