Description
Vitamin D is either produced in the skin (under the influence of UV light) or supplied through the diet.
The storage type of vitamin D, namely 25-OH-vitamin D, is formed in the liver. The hormone 1,25-(OH)2 -vitamin D (D hormone) is formed in a second hydroxylation step in the kidney. The responsible enzyme, 1α-hydroxylase, is subjected to a rigid control through hormones (especially parathyroid hormone) and its activity is influenced by the serum concentrations of calcium and phosphate. The serum concentration of 1,25-(OH)2 -vitamin D normally re-adjusts itself to the demands of metabolism. Deviations from the normal range of 1,25-(OH)2 – vitamin D must therefore always be interpreted in the context of the remaining parameters of the calcium metabolism. The serum concentration of 1,25-(OH)2 – vitamin D decreases only in seldom cases of vitamin D deficiency. For the diagnosis of vitamin D deficiency, the precursor metabolite, 25-OH-vitamin D, should be measured. Supplemental vitamin D is available in two distinct forms: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). The determination of 1,25-(OH)2 -vitamin D3 /D2 as a measure of 1,25-(OH)2 – vitamin D status provides an objective, quantitative measure of the biological response to vitamin D administration.