Use of Vitamin D Binding Protein and 25-Hydroxyvitamin D Levels in Clinical Practice for Endocrinologists.

Currently, the total 25-hydroxyvitamin D (25(OH)D) is recognized as the indicator of vitamin D status that is used to define vitamin D sufficiency. Vitamin D binding protein (DBP) is the major carrier for circulating vitamin D and plays an important role in regulating circulating total and free vitamin D metabolites. Since the concentration of DBP and its affinity to vitamin D varies by different physiologic and clinical conditions, measuring total 25(OH)D may not accurately reflect functional vitamin D status. In addition, DBP is a potential prognostic indicator of clinical outcomes since it has other important functions beyond that as vitamin D carrier, including its role in actin scavenging after tissue injury and inflammation and immune modulation. It has been proposed that circulating DBP is altered in some clinical conditions, which affects the levels of total and free vitamin D metabolites and can explain clinical outcomes. Furthermore, in some clinical situations, total 25(OH)D levels are altered and knowing whether DBP is also changed may have diagnostic and therapeutic implications. The goal of this review is to assess clinical conditions altering DBP concentrations and then total 25(OH)D levels and their effects on prognosis. We suggest using the free 25(OH)D level as a better marker for vitamin D status in certain clinical conditions.