Vitamin D

• Reference Range-Sufficiency:  30--100 ng/mL;
• Deficiency <20 ng/mL;
• Insufficiency 20-30 ng/mL;
• Toxicity >100 ng/mL

　A vitamin D test is used to:

• Determine if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium , phosphorus, PTH) is occurring as a result of a deficiency or excess of vitamin D
• Help diagnose or monitor problems with parathyroid gland functioning since PTH is essential for vitamin D activation
• Screen people who are at high risk of deficiency, as recommended by the National Osteoporosis Foundation, the Institute of Medicine, and the Endocrine Society
• Help monitor the health status of individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn disease, since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat
• Monitor people who have had gastric bypass surgery and may not be able to absorb enough vitamin D
• Help determine the effectiveness of treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation is prescribed

There are two major types of vitamin D:

1. Vitamin D₂ (ergocalciferol)- Synthesized by plants and mushrooms, and is not produced by the human body. The major preparations of vitamin D for prescription use in North America are in the form of vitamin D₂.
2. Vitamin D₃ (cholecalciferol)- Made in large quantities in the skin from the conversion of 7-dehydrocholesterol in the epidermis and dermis when sunlight strikes bare human skin. It can also be ingested from oil-rich fish (e.g. salmon, mackerel, and herring, egg yolks, foods supplemented with vitamin D and other animal sources. There is evidence that vitamin D₃ is approximately three times more effective at maintaining serum concentrations because the binding protein has a higher affinity to vitamin D₃ than vitamin D₂, which allows vitamin D₃ to reside longer and increase the concentration to sufficient levels more quickly. Over-the-counter vitamin/multivitamin preparations mainly use vitamin D₃.

   Prescription or over-the-counter dietary supplements are a major source of vitamin D for many people. Factors such as latitude, time of the day, aging, increased skin pigmentation, ethnic origin, application of sunscreen and season of the year can dramatically affect the production of vitamin D3 in the skin and thus the levels of vitamin D in the blood. Vitamin D deficiency can result from inadequate exposure to the sun, inadequate alimentary intake, decreased absorption, abnormal metabolism, or vitamin D resistance. Vitamin D originating from the skin or the diet is biologically inactive. Whether consumed or produced, both forms of vitamin D (D2 and D3)are bound to a binding protein (albumin and vitamin D binding protein), It enters the circulation bound to this vitamin D binding protein (DBP), and is transported to the liver to undergo a hydroxylation to produce 25(OH) vitamin D (which also circulates as a complex with DBP). Carried through the blood stream to the liver, it is metabolized by the liver to 25 (OH)-vitamin D (calcidiol), and then It is further metabolized in the kidneys by the enzyme 25-hydroxy vitamin D-1α-hydroxylase to its biologically active form, 1,25-dihydroxyvitamin D (calcitrol). 1,25-dihydroxyvitamin D circulates at levels 1000 times lower than 25(OH) vitamin D and its renal production is tightly regulated by plasma parathyroid hormone levels and serum calcium and phosphorus levels. Serum 25(OH) vitamin D is the major circulating metabolite of vitamin D in the body and reflects vitamin D inputs from cutaneous synthesis and dietary intake. For this reason, serum concentration of 25(OH) vitamin D is considered the standard clinical measure of vitamin D status. Because serum 25(OH) vitamin D will be a mixture of the D2 and D3 forms, both the vitamin D2 and vitamin D3 forms of vitamin D must be measured to accurately assess total 25(OH) vitamin D levels. Vitamin D metabolites are bound to a carrier protein in the plasma and distributed throughout the body. The most reliable clinical indicator of vitamin D status is 25(OH) Vitamin D because serum and plasma 25(OH) Vitamin D levels reflect the body’s storage levels of Vitamin D, and 25(OH) Vitamin D correlates with the clinical symptoms of Vitamin D deficiency.