Stress axis (HPA)
The stress axis: three tiers, one brake.
Stress in, cortisol out: glucose mobilized, inflammation damped
This mnemonic, unpacked word by word →
The same three-tier architecture as the reproductive axis, aimed at survival: the hypothalamus whispers CRH, the pituitary relays ACTH, the adrenal cortex answers with cortisol, and cortisol then silences both tiers above it. That brake is the exam's favorite lever: block it, remove it, or exhaust it and predict the labs.
CRH arrives in a circadian rhythm that peaks before dawn, amplified by physical or psychological stress. ACTH is cut from the POMC precursor, the same protein that yields melanocyte-stimulating hormone, which becomes the pigmentation clue in Addison disease. The anterior pituitary's full roster: FLAT PEG. FSH, LH, ACTH, TSH are tropic; Prolactin, Endorphins, GH act directly. ("GOAT FLAP" folds in the posterior pair as the O and A: Oxytocin and ADH.)
A stimulus rather than a conversion: ACTH tells the zona fasciculata to build cortisol from cholesterol. Aldosterone lives one zone out (glomerulosa) and answers to angiotensin II and K⁺, with ACTH playing no part.
Cortisol shuts off its own supply line at both tiers. The dexamethasone test reads this brake: a synthetic cortisol that suppresses ACTH in a healthy axis but not in an autonomous tumor. Long steroid therapy pushes the same brake for months. Stop abruptly and the atrophied axis cannot restart.