Scandinavian journal of clinical and laboratory investigation
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Scand. J. Clin. Lab. Invest. · Apr 2014
Serum free and bio-available 25-hydroxyvitamin D correlate better with bone density than serum total 25-hydroxyvitamin D.
In the circulation 25-hydroxyvitamin D (25(OH)D) is bound to vitamin D-binding protein (DBP) and albumin. Only a small fraction is in the unbound, free form. According to the 'free-hormone-hypothesis' only the free form is biologically active. ⋯ These relations were almost exclusively seen in subjects not using vitamin D and/or calcium supplements. In conclusion, the free and bio-available forms of 25(OH)D may be a more informative measure of vitamin D status than total 25(OH)D. Adjustment for DBP phenotype may improve this further.