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-hydroxyvitamin D, is formed in the liver. The hormone 1,25-dihydroxyvitamin D (D hormone) is formed in a second hydroxylation step in the kidney. The responsible enzyme, the kidney 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-dihydroxyvitamin D normally re-adjusts itself to the demands of metabolism. Deviations from the normal range of 1,25-dihydroxyvitamin D must therefore always be interpreted in the context of the remaining parameters of the calcium metabolism. The serum concentration of 1,25-dihydroxyvitamin D decreases only in seldom cases of vitamin D deficiency. For the diagnosis of vitamin D deficiency, the precursor metabolite, 25-hydroxyvitamin D, should be measured.
The reason for a non-physiological deficiency of 1,25-dihydroxyvitamin D can be found in metabolic disturbances, such as kidney malfunctions, or by genetic defects of the enzyme 1α-hydroxylase. Even a slightly impaired kidney function can lead to a decrease of the 1,25-dihydroxyvitamin D concentration.
