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.
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Roux et al. (2002) studied this question.
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