Category: endocrine_axis
Stress + circadian cortisol regulation. Hypothalamic CRH → anterior pituitary ACTH → adrenal cortex (zona fasciculata) cortisol. Negative feedback: cortisol → hypothalamus + pituitary (suppresses CRH + ACTH). Exogenous glucocorticoids suppress the entire axis — chronic high-dose Rx (>10-20 mg/d prednisone equivalent for >2-3 weeks) produces tertiary adrenal insufficiency that requires gradual taper. ACTH-stim test diagnoses primary vs secondary AI. Diurnal rhythm: ACTH + cortisol peak in early morning (~6-8am), trough at night (~midnight) — the basis for AM cortisol measurement and dexamethasone suppression test.