This test measures the level of Vitamin D3 in blood. Vitamin D deficiency leads to bone malformation, softness or fracture of the bone and abnormal levels of calcium. 25- OH vitamin D3 test is carried out to determine vitamin D deficiency and vitamin D supplementation level. Vitamin D exists in two forms in the blood, 25- hydroxyl D [25(OH) D] and 1, 25-dihydroxy vitamin D [1, 25 (OH) (2) D]. 25- Hydroxy vitamin D is the major form of the hormone found in the blood and is vital for the growth and health of the bone. Without it bones turn soft resulting in various diseases such as rickets and osteomalacia. It also regulates the absorption of calcium, phosphorus, and to a lesser extent, magnesium.