Save 20% on the Vitamin D, 25-Hydroxy, Total, Immunoassay Lab Test. Measurement of serum 25-OH vitamin D concentrations provide a good index of circulating vitamin D activity in patients not suffering from renal disease. Lower than normal 25-OH vitamin D levels can result from a dietary deficiency, poor absorption of the vitamin or impaired metabolism of the sterol in the liver. A 25-OH vitamin D deficiency can lead to bone diseases such as rickets and osteomalacia. Above normal levels can lead hypercalcemia. - This test measures the total level of vitamin D in the blood and the components D2 & D3. Vitamin D comes from two sources: endogenous, which is produced in the skin on exposure to sunlight, and exogenous, which is ingested in foods and supplements. The types of vitamin D are slightly different, and they are named vitamin D2 ( which comes from plants) and vitamin D3 (which comes from animals). The D2 form is found in fortified foods and most vitamin preparations and supplements. Vitamin D3 is the form produced in the body and is also in some supplements.