Not all organs are equally capable of absorbing vitamin D metabolites. This phenomenon is partly responsible for the fact that to ensure the various effects of vitamin D (endocrine or autocrine/paracrine), different “optimal” 25(OH)D serum concentrations may be required. The kidney, in particular, holds a special position. According to current knowledge, it is the only one of our numerous organs that has special mechanisms (so-called megalin/cubilin system) that enable it to produce and release sufficient amounts of biologically active 1,25-dihydroxyvitamin into the blood, even at relatively low 25(OH)D levels in the blood (greater than about 10 ng/ml). In contrast, the other organs require significantly higher 25(OH)D levels in the blood (greater than about 20 ng/ml) for sufficient synthesis of 1,25-dihydroxyvitamin D for autocrine/paracrine effects.