Biological or Clinical Significance:
Vitamin D, also known as caliciferal; is a group of fat-soluble secosteroids, the two major physiologically revlevant forms of which are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D without a subscript refers to either D2 or D3 or both.
Vitamin D3 is derived mainly from actions of ultraviolet light on the skin while D2 is derived solely from dietary sources. Since these two parent compounds provide various contributions to the overall vitamin D status of the individual, it is important that both forms are measured equally.
As (Vitamin D) enters the circulation, it is metabolized to several forms, the majority of these being 25-hydroxycalciferol (25-OH-D). The first step in the metabolism of vitamin D, 25-hydroxylation, occurs mainly in the liver. Only a small amount of 25-OH-D is metabolized in the kidney to other dihydroxyvitamin D metabolites. Since 25-OH-D is the predominant circulating form of vitamin D in the normal population, it is considered to be the most reliable index of vitamin D status.
The active form of the vitamin, 1,25-dihydroxyvitamin D, is formed in the kidney under the combined regulation by parathyroid hormone and calcium and phosphorous nutritional intake. The dihydroxy form of the vitamin apparently has roles in regulation of calcium and phosphorous metabolism and a variety of other physiologic processes.
Principle of Test Method:
The 1,25-(OH)2 –Vitamin D assay is a competitive radioimmunoassay with sample extraction.
Freedman DM, Tangrea JA, Virtamo J, Albanes D. The effect of beta-carotene supplementation on serum vitamin D metabolite concentrations. Cancer Epidemiol Biomarkers Prev. 1999; 8:1115-1116
Hollis BW, Kamerud JQ, Kurkowski A, Beaulieu J, Napoli JL. Quantification of circulating 1,25-dihydroxyvitamin D with an 125I-labeled tracer. Clin Chem 1996; 42:596-592
Endres, DB, Rude, RK. Mineral and bone metabolism. in Tietz Textbook of Clinical Chemistry 3rd Ed., Burtis CA, Ashwood, ER, eds. Saunders, Philadelphia, 1999,1395 - 1457.