The HPA axis is a communication chain between three glands: the hypothalamus and pituitary gland in your brain, and the adrenal glands on top of your kidneys. Together, they control your body’s production of cortisol, the hormone most associated with stress. When something threatens or challenges you, these three glands signal each other in a rapid sequence that raises cortisol levels, then shut the process down once the threat passes. It’s one of the most important regulatory systems in the body, influencing everything from blood sugar and blood pressure to immune function and memory.
The Three Glands and What They Do
The name “HPA axis” is shorthand for the hypothalamic-pituitary-adrenal axis, named for each gland in the chain.
The hypothalamus sits deep inside the brain and acts as a control center for maintaining balance across the body. It monitors temperature, hunger, mood, and dozens of other internal signals. When it detects a stressor, whether physical danger, illness, or psychological pressure, it kicks off the hormonal cascade by releasing a signaling hormone called CRH (corticotropin-releasing hormone).
The pituitary gland is a pea-sized gland at the base of the brain, just below the hypothalamus. Its front portion (the anterior pituitary) receives the CRH signal and responds by releasing ACTH (adrenocorticotropic hormone) into the bloodstream.
The adrenal glands are small, triangle-shaped glands that sit on top of each kidney. When ACTH reaches them, the outer layer of the adrenal glands (the adrenal cortex) produces and releases cortisol.
How the Signaling Cascade Works
The HPA axis operates like a relay. A stressful event triggers the hypothalamus to release CRH, which tells the pituitary to release ACTH, which tells the adrenals to release cortisol. This whole sequence begins within minutes. ACTH pulses tend to be brief, while cortisol levels rise more gradually and stay elevated longer, depending on how intense or prolonged the stressor is.
Equally important is how the system turns itself off. Once cortisol levels climb high enough, cortisol itself signals back to the hypothalamus and pituitary to stop producing CRH and ACTH. This is called a negative feedback loop, and it’s essential for preventing cortisol from staying elevated indefinitely. At the molecular level, cortisol activates receptors that physically block the signals needed to keep CRH and ACTH production going. When this feedback loop works properly, cortisol rises to meet a challenge and then falls back to baseline once the challenge is over.
What Cortisol Actually Does in the Body
Cortisol often gets labeled “the stress hormone,” but its job description is much broader. It’s a survival hormone, designed to make sure your body has the resources it needs during a crisis.
One of cortisol’s primary roles is raising blood sugar. It stimulates the liver to produce new glucose and simultaneously slows glucose absorption in other tissues, making more energy available for muscles and the brain. It also affects fat and protein metabolism, breaking down stored energy reserves when needed. Beyond metabolism, cortisol helps maintain blood pressure and blood volume, suppresses inflammation, and dampens immune activity. These effects are useful in the short term: during acute stress, you don’t want your immune system wasting energy on a low-grade infection when you need to respond to an immediate threat.
Cortisol also affects the brain directly, influencing mood, alertness, and memory formation. Stressful or emotionally intense experiences tend to be well-remembered partly because stress hormones, including cortisol, enhance the brain’s ability to lock in memories during those moments.
The Daily Cortisol Rhythm
The HPA axis doesn’t just respond to stress. It also follows a predictable daily cycle. Cortisol levels are lowest around midnight and begin climbing in the early morning hours. Within the first 30 minutes after waking, cortisol surges by roughly 50% in a pattern called the cortisol awakening response. This morning spike helps you feel alert and ready for the day. Levels then gradually decline through the afternoon and evening.
This circadian rhythm means that the same cortisol level can be normal at 8 a.m. and abnormal at 10 p.m. It also means that disruptions to sleep, shift work, or chronic stress can throw off the timing of the entire cycle, not just the peak levels.
Acute Stress vs. Chronic Stress
The HPA axis handles a brief, intense stressor very differently from ongoing, unrelenting pressure. During acute stress, the system fires up quickly, cortisol rises, and once the stressor ends, feedback mechanisms bring everything back to normal. This is the system working exactly as designed.
Chronic stress is another story. When the HPA axis is activated repeatedly over weeks or months, the system can start to malfunction in several ways. Some people develop chronically elevated baseline cortisol. Others show exaggerated cortisol responses to new stressors. In some cases, the adrenal glands can become overly sensitive to ACTH, amplifying the cortisol response even further. The brain circuits involved in chronic stress responses can also shift: the neural pathways driving the HPA axis under chronic stress are not always the same ones involved in acute reactions. The body essentially rewires parts of the stress system.
The consequences of prolonged HPA axis overactivation are wide-ranging. Chronically high cortisol can increase inflammation (paradoxically, since cortisol is anti-inflammatory in the short term), weaken the gut lining, suppress immune defenses, and contribute to metabolic problems like insulin resistance. These effects help explain why chronic psychological stress is linked to conditions ranging from inflammatory bowel disease to cardiovascular problems.
How the HPA Axis Works With Other Stress Systems
The HPA axis doesn’t work alone. It operates alongside the sympathetic nervous system, which is responsible for the immediate “fight or flight” response: the racing heart, rapid breathing, and adrenaline surge you feel within seconds of a threat. While the sympathetic system acts almost instantly through nerve signals, the HPA axis works on a slightly slower timeline through hormones in the bloodstream. The two systems reinforce each other. Sympathetic activation enhances the adrenal glands’ sensitivity to ACTH, making cortisol production more efficient during stress. Meanwhile, cortisol and adrenaline work together to strengthen memory formation, which is why emotionally charged events tend to stick in your mind.
HPA Axis Dysfunction and “Adrenal Fatigue”
If you’ve come across the term “adrenal fatigue,” it’s worth knowing that this is not a recognized medical diagnosis. The idea, popular in alternative health circles, is that chronic stress literally wears out the adrenal glands until they can no longer produce enough cortisol. While the concept captures something real about how people feel after prolonged stress (exhaustion, brain fog, difficulty coping), the mechanism it describes doesn’t match what researchers actually observe.
What does happen is HPA axis dysregulation: changes in the timing, magnitude, or feedback sensitivity of the cortisol response. The adrenal glands themselves are rarely the problem. Instead, the issue is usually in the signaling between the brain and the glands, or in how the body responds to cortisol at the cellular level. This distinction matters because the treatments are different. True adrenal insufficiency, where the glands genuinely cannot produce enough cortisol, is a serious medical condition diagnosed with a stimulation test. In that test, synthetic ACTH is injected and cortisol levels are measured afterward. A cortisol response below roughly 15 to 18 micrograms per deciliter (the exact cutoff depends on the lab’s assay) suggests the adrenals aren’t functioning properly. Baseline cortisol below 3 micrograms per deciliter is strongly suggestive of insufficiency, while levels above 10 generally indicate normal function.
HPA axis dysregulation from chronic stress, by contrast, doesn’t usually show up as frank adrenal insufficiency on testing. It’s a subtler pattern of mistimed or mismatched cortisol output that standard diagnostic thresholds may not catch, which is part of why it’s a more complex clinical picture to address.