Human 25 hydroxyvitamin D is also known as Calcifediol or Cacidiol. This is a prehormone produced by the liver. It is produced by vitamin D3 hydroxylation by the enzyme cholecalciferol 25 hydroxylase. Michael F. Holick was first to isolate it. Vitamin D status of patients can be determined by measuring this metabolite and this method is widely used by physicians. Human 25 hydroxyvitamin D is converted in the kidneys with the aid of the enzyme 25(OH)D-1alpha-hydroxylase into calcitriol. This is a secosteroid hormone which is an active form of Vitamin D.  Determination of Vitamin D in the body is aided by using blood test measuring the levels of 25- hydroxy vitamin D and this is the best indicator of Vitamin D status in patients. Vitamin D defieciencies can be detected by performing this test.

Obese patients, patients with malabsorption, osteoporosis and kidney diseases are at higher risk for vitamin D defieciencies. Individuals residing in the areas with limited sun exposures show a higher risk of vitamin D defieciencies. The normal range of 25 – hydroxy vitamin D is 30 to 74ng/ml but this range varies according to age and geographical location. Increased levels of 25-hydroxy vitamin D are related to increased absorption of calcium from the gut. Deficiencies of 25- hydroxy vitamin D lead to Rickets, osteomalacia, and cardiovascular diseases. Vitamin D supplementation in older people with osteoporosis prevents fractures but also increases stomach and kidney issues. Serum levels of 25- hydroxy vitamin D above 50ng/ml can lead to various concerns. Risk of cardiovascular diseases is lower in cases where the vitamin D concentration is in the range of 20-60 nmol/l.