Vitamin D is a fat-soluble precursor of the steroid hormone calcitriol, which is required to maintain normal blood calcium and phosphate levels. It plays a critical role in several metabolic functions, including bone mineralization, muscle contraction, nerve conduction, and overall cellular functioning. Total Vitamin D assays measure the level of both 25 OH Vitamin D3 (calcidiol) and 1,25-Dihydroxy Vitamin D3 (calcitriol). Serum 25 OH Vitamin D3 is the most stable circulating form of vitamin D, and a low blood level suggests that a person is not getting enough exposure to sunlight or enough dietary vitamin D. 1,25-Dihydroxy Vitamin D3 regulates the expression of hundreds of genes and a low level is indicative of kidney failure.