Free cortisol index as a surrogate marker for serum free cortisol

Background The biologically active component of a hormone is the unbound or free fraction. Changes in cortisol-binding protein could give misleading results if only total cortisol is measured for the interpretation of dynamic function tests. Methods This study aimed to measure serum free cortisol using a steady-state gel-filtration method and then to evaluate the correlation between the serum free cortisol and the free cortisol index (FCI), defined as serum total cortisol/cortisol-binding globulin (CBG). Results Forty-eight serum samples from healthy volunteers undergoing a short Synacthen test were analysed for total cortisol, free cortisol and CBG. The FCI correlated well with a previously established, but more complex, calculation of serum free cortisol (R=0.98, P<0.001) and with measured serum free cortisol (R=0.90, P<0.001). Conclusion Free cortisol index is a reliable and user-friendly measure of serum free cortisol.