Vitamin D metabolism and action
There are two natural forms of vitamin D: cholecalciferol (D3) and ergocalciferol (D2). Humans acquire vitamin D by two routes—endogenous synthesis in the skin during sunlight exposure (D3) and dietary intake (D2 and D3). Vitamin D from either source is converted in the liver by 25-hydroxylase to 25-hydroxy (OH) vitamin D and then by 1-alpha-hydroxylase in the kidney to 1,25-dihydroxy (OH) 2 vitamin D. The latter binds to intestinal vitamin D receptors to promote calcium and phosphorous absorption. Parathyroid hormone (PTH) and hypophosphatemia are major inducers of 1 alpha-hydroxylase activity. As a patient becomes vitamin D-deficient and serum calcium decreases, the PTH level increases inducing the 1-alpha-hydroxylase enzyme to convert 25-OH vitamin D into 1,25 (OH) 2 vitamin D. Therefore, the 25-OH vitamin D level will decrease before the 1,25 (OH) 2 vitamin D level. Therefore, measuring 25-OH vitamin D level is the best measure of vitamin D stores.