Constraints, concerns and considerations about the necessity of estimating free glucocorticoid concentrations for field endocrine studies

We evaluate the utility of measuring corticosteroid-binding globulins (CBG) and estimating free glucocorticoid (GC) concentrations for field endocrine studies. We assert that for three general reasons, measurement of free GCs might not be more useful than total GCs for many studies. First, estimates of so-called free' GCs are likely inaccurate, in part because of the following: (i) other factors in the blood also bind GCs, (ii) CBG binds plasma steroids other than GCs and (iii) CBG binding affinity can vary with local conditions, such as enzymatic activity and tissue temperature. Second, evidence suggests an active role for CBG-bound GCs, CBG or both, in the vertebrate stress response, calling into question the validity and generality of the free hormone hypothesis. Third, free and total GCs function over different time frames. Free GCs are likely important in the seconds-to-minutes time-scale of interaction with tissues, but total GCs could function at minutes-to-hours time-scales by serving as the reservoir to continue supplying GCs to tissues. As transcription regulators, most GC effects manifest in hours; thus, total GCs would be the most appropriate measure for estimating total biological impact. Our understanding of the biochemistry and the biological actions of both GCs and CBG indicates that total GC concentrations are currently less prone to error and more biologically interpretable than estimates of free hormone. Although further work is necessary, total GC titres currently remain the most accurate and informative estimates of stress hormone levels to address biological questions in nature. Lay Summary