Cortisol Axis Testing

Cortisol production follows a three-level signaling pathway. Hypothalamic corticotropin-releasing hormone (CRH) stimulates pituitary release of adrenocorticotropic hormone (ACTH), and ACTH stimulates the adrenal cortex to produce cortisol. Rising cortisol then feeds back to reduce CRH and ACTH release. Testing the cortisol axis asks one of two very different questions: - Suppression testing asks whether excess cortisol production can be turned off by a feedback signal. It is used when Cushing syndrome or autonomous adrenal cortisol production is suspected. - Stimulation testing asks whether the adrenal cortex can increase cortisol output when given ACTH. It is used when adrenal insufficiency is suspected. The distinction prevents a common error: dexamethasone suppression is not a test for adrenal insufficiency, and cosyntropin stimulation is not a screening test for Cushing syndrome.