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The HPA Axis: How Stress Reaches Your Adrenals

From a worried thought to a surge of cortisol in three relay stations. Trace the hypothalamic–pituitary–adrenal axis and the feedback that switches it off again.

A three-station relay

When biologists talk about “the stress hormone system,” they almost always mean the hypothalamic–pituitary–adrenal (HPA) axis. An endocrine axis is simply a chain of glands that pass a signal down a line, each amplifying it for the next. The HPA axis has three stations: the hypothalamus at the top, the anterior pituitary in the middle, and the adrenal cortex at the bottom.

  1. A stressor — physical or psychological — excites parvocellular neurons in the hypothalamus to release CRH (corticotropin-releasing hormone) into the portal blood that runs to the pituitary.
  2. CRH tells the anterior pituitary to release ACTH into the general circulation.
  3. ACTH reaches the adrenal cortex and drives it to make cortisol, the body’s main glucocorticoid.
  4. Cortisol floods the body, raising blood sugar, sharpening attention, and dampening processes that can wait — such as inflammation, growth and reproduction.

The off-switch: negative feedback

An axis that could only switch on would be a disaster. The HPA axis turns itself off through negative feedback: the cortisol at the bottom of the chain travels back up and tells the top two stations to quiet down. High cortisol suppresses CRH from the hypothalamus and ACTH from the pituitary, which lowers cortisol, which lifts the suppression — a self-correcting loop that keeps the hormone near its set point.

HPA AXIS — signal down, feedback up

  stressor
     |
     v
  HYPOTHALAMUS --(CRH)--> ANTERIOR PITUITARY --(ACTH)--> ADRENAL CORTEX
     ^                          ^                              |
     |                          |                          (cortisol)
     +----- negative feedback --+------------------------------+

  cortisol HIGH  -> CRH & ACTH suppressed -> cortisol falls
  cortisol LOW   -> suppression lifts      -> cortisol rises
  net effect: cortisol held near its set-point
Signal flows down the three stations; cortisol feeds back up to throttle the top two.

A daily rhythm, not just an alarm

The HPA axis is not only a fire alarm. Even on a calm day cortisol rises and falls in a diurnal pattern: lowest around midnight, peaking in the early morning to help you wake and meet the day. That morning jump has its own name, the cortisol awakening response. Understanding both modes — the steady daily rhythm and the sharp stress spike — is the key to everything that follows about chronic stress.