Vitamin D is a fat-soluble prohormone with several active metabolites that act as hormones. It is metabolized by the liver to 25(OH)D, which is then converted by the kidneys to 1,25(OH)2D (1,25-dihydroxycholecalciferol, or active vitamin D hormone). 25(OH)D, the major circulating form, has some metabolic activity, but 1,25(OH)2D is the most metabolically active. The conversion to 1,25(OH)2D is regulated by its own concentration, parathyroid hormone (PTH), and serum concentrations of Ca and phosphate. Vitamin D affects many organ systems but mainly it is required for calcium absorption, bone metabolism, regulation of cell development and the immune system. Sources include: dairy, egg yolks, fish oils, fortified foods; also synthesized in response to ultraviolet radiation. This test is used to detect vitamin D deficiency or poisoning (due to excessive intake). The levels of 25(OH)D give a measure of overall reserves, dietary intake and endogenous sunlight-dependent synthesis. 1,25(OH)2D is a metabolite of vitamin D and measuring its levels is useful in investigating disorders which might be related to vitamin D metabolism. Populations who may be at a high risk for vitamin D deficiencies include the elderly, obese individuals, exclusively breastfed infants, and those who have limited sun exposure. Also, individuals who have fat malabsorption syndromes (e.g., cystic fibrosis) or inflammatory bowel disease (e.g., Crohns disease) are at risk. Vitamin D DEFICIENCY impairs bone mineralization, causing rickets in children and osteomalacia in adults and possibly contributing to osteoporosis. It also causes hypocalcemia, which stimulates production of PTH, causing hyperparathyroidism. Deficiency may also be associated with cardiovascular disease, cancer (colon, breast), and autoimmune diseases. Symptoms include muscle aches, muscle weakness, and bone pain. The main symptoms associated with vitamin D TOXICITY result from hypercalcemia; Anorexia, nausea, and vomiting can develop, often followed by polyuria, polydipsia, weakness, nervousness, pruritus, and eventually renal failure. Proteinuria, urinary casts, azotemia, and metastatic calcifications (particularly in the kidneys) can also develop.