<qa>  What is the name of the protein bound fraction that is taken up by megalin?Vitamin-D is a protein-bound steroid hormone and interpretation of vitamin-D surgical studies require knowledge about the effect of clinical surgical trauma per se on vitamin-D physiology, as has been demonstrated with cortisol, thyroid hormones and other steroid hormones Kehlet, Brandt, Rem. Unlike other steroid hormones vitamin D depends on a two step activation process before it is able to bind and activate the vitamin D receptor VDR. First by 25-hydroxylation CYP2R1 in the liver, which forms 25-hydroxyvitamin D 25-OHD. 25-OHD is also biologically inactive but used clinically to determine vitamin D status because serum 25-OHD levels are associated with rickets, bone health and calcium homeostasis. Circulating 25-OHD is bound to vitamin D binding protein DBP Prosser and jones 2004. This complex is filtered in the glomerulus and subsequently transported into the proximal tubules cells by megalin-mediated uptake Nykjær 1999. Here, 25-OHD undergoes 1α-hydroxylation CYP27B1 and the resulting 1,25OH2D3 binds and activates the vitamin D receptor VDR. The fact that the protein bound fraction is taken up by megalin differs from the metabolic clearance andor activation of other steroid hormones and questions whether the free hormone hypothesis is applicable to the vitamin D system. The fact that binding proteins are lost during surgical stress indicates that this patient group is ideal for studying total versus free hormone concentrations and possible secondary changes for instance in serum PTH and calcium due to functional hormonal aberrations. 