Free 25 (OH) vitamin D incl. VDBP (Vit. D binding protein)

Serum, 1ml

Only free vitamin D is biologically active. About 1 % of the total vitamin D is present in free form. The remaining 99 % is mainly bound to the vitamin D-binding protein (VDBP) and to a lesser extent to albumin and lipoproteins. The bound vitamin D is biologically inactive and serves as a reservoir for the body. The proportion of free vitamin D in total vitamin D varies from person to person and depending on the clinical situation it is influenced by the level of VDBP and its binding affinity to vitamin D.

VDBP and albumin are produced in the liver. The production of these proteins is affected accordingly in the case of liver dysfunction or liver disease, so that the measurement of total vitamin D, for example, could show a deficiency, whereas there would actually be sufficient free, biologically active vitamin D. Sex hormones, especially oestrogens, stimulate VDBP production, which means that free vitamin D is bound more. Proteinuria due to kidney disease leads to an outflow of vitamin D bound to VDBP. In addition, the production and binding affinity of VDBP seems to be influenced by genetic factors.

It therefore makes sense to determine the free vitamin D instead of the total vitamin D, especially for pregnant women and in cases of liver and kidney disease. If the total vitamin D in patients increases insufficiently after vitamin D substitution, the analysis of the free vitamin D is also useful.