Journal Article

Regulation of cortisol bioavailability—effects on hormone measurement and action

Trainer, Peter J., Perogamvros, Ilias, Ray, David W.

2012

Overview

Clinical assessment of the hypothalamic–pituitary–adrenal axis relies on measuring total serum cortisol levels. However, most circulating cortisol is bound to carrier proteins and only free cortisol is biologically available. This Review examines factors that regulate peripheral and tissue glucocorticoid bioavailability and highlights specific clinical indications for the measurement of bioavailable glucocorticoids. Routine assessment of the hypothalamic–pituitary–adrenal axis relies on the measurement of total serum cortisol levels. However, most cortisol in serum is bound to corticosteroid-binding globulin (CBG) and albumin, and changes in the structure or circulating levels of binding proteins markedly affect measured total serum cortisol levels. Furthermore, high-affinity binding to CBG is predicted to affect the availability of cortisol for the glucocorticoid receptor. CBG is a substrate for activated neutrophil elastase, which cleaves the binding protein and results in the release of cortisol at sites of inflammation, enhancing its tissue-specific anti-inflammatory effects. Further tissue-specific modulation of cortisol availability is conferred by corticosteroid 11β-dehydrogenase. Direct assessment of tissue levels of bioavailable cortisol is not clinically practicable and measurement of total serum cortisol levels is of limited value in clinical conditions that alter prereceptor glucocorticoid bioavailability. Bioavailable cortisol can, however, be measured indirectly at systemic, extracellular tissue and cell levels, using novel techniques that have provided new insight into the transport, metabolism and biological action of glucocorticoids. A more physiologically informative approach is, therefore, now possible in the assessment of the hypothalamic–pituitary–adrenal axis, which could prove useful in clinical practice. Key Points Only the unbound (free) fraction of cortisol is biologically active Current hypothalamic–pituitary–adrenal axis assessment relies on measurement of total serum cortisol levels, which are a surrogate for free serum cortisol concentrations Prereceptor regulation of cortisol involves binding to corticosteroid-binding globulin and cortisol–cortisone interconversion, which both affect cortisol bioavailability for the glucocorticoid receptor Various conditions and medications affect protein binding and total, but not bioavailable, cortisol levels; total serum cortisol levels are inappropriately used as a biochemical end point and can be misleading Novel methodologies have led to a transition from total serum cortisol measurement by immunoassays to free serum cortisol measurement by liquid chromatography and tandem mass spectrometry Tissue microdialysis and glucocorticoid bioassays could be used in the future to clarify the relationship of circulating to tissue and cell levels of available cortisol

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/443/163 / 631/443/319/367/1562 / 631/92/436/1729 / 692/700/139 / Animals / Bioavailability / Biological Availability / Biological control systems / Carrier proteins / Clinical medicine / Endocrinology / Hormones / Humans / Hydrocortisone / Hydrocortisone - blood / Hypothalamic-pituitary-adrenal axis / Hypothalamo-Hypophyseal System - metabolism / Hypotheses / Measurement / Medicine / Medicine & Public Health / Physiological aspects / Pituitary-Adrenal System - metabolism / Protein Binding - physiology / Proteins / Receptors, Glucocorticoid - metabolism / review-article / Steroids / Transcortin - metabolism