25-OH Vitamin D

SST

Serum (gold or red top) Tube

Daily upon receipt

Vitamin D is a fat soluble steroid hormone that comes in two forms, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).  Vitamin D is synthesized from cholesterol upon skin exposure to UVB sunlight or through dietary intake.  Vitamin D is hydroxylated in the liver to form 25-OH Vitamin D which is further hydroxylated in the kidney to form the biologically active form, 1,25-(OH)2 Vitamin D.  The active hormone is tightly regulated by plasma parathyroid hormone levels and serum calcium and phosphorous levels.  The active form, 1,25-(OH)2 Vitamin D, increases the intestinal absorption of calcium and phosphorous, both are required for regulating bone metabolism.  Vitamin D metabolites are bound to a vitamin D binding protein and are circulated throughout the body.  The concentration of 1,25-(OH)2 Vitamin D is 1000 times lower than 25-OH Vitamin D and has a half life of 4 hours.  Due to its half life of 2–3 weeks, 25-OH Vitamin D is the metabolite that is the most reliable clinical indicator of vitamin D status.  Also, 25-OH Vitamin D levels are indicative of the body’s storage levels of vitamin D and correlate with the clinical symptoms of vitamin D deficiency. In the late 18th century, vitamin D was first recognized as an essential dietary component in the prevention of rickets.  Recently, research has indicated that vitamin D deficiency may be linked to chronic diseases such as cancer (breast, colon and prostate), cardiovascular disease, osteoporosis, osteomalacia and several autoimmune diseases among others.