HPA axis: the body’s stress chain, explained

The HPA axis (hypothalamic-pituitary-adrenal axis) is the hormone pathway your brain and adrenal glands use to manage stress. Your hypothalamus signals your pituitary gland, which signals your adrenals to release cortisol. It’s slower than an adrenaline jolt, built for sustained demands, and a feedback loop normally shuts it back down once the stressor passes.

How does the HPA axis actually work?

Your hypothalamus, a small structure at the base of your brain, starts the chain. When it registers a demand or threat, it releases corticotropin-releasing hormone (CRH). CRH travels to your pituitary gland and prompts it to release adrenocorticotropic hormone (ACTH). ACTH then travels through your bloodstream to your adrenal glands, two small glands that sit above your kidneys, and signals them to release cortisol.

Cleveland Clinic describes this three-step relay as the body’s main system for managing a stress response. The StatPearls physiology reference on cortisol notes that this same axis also governs how much cortisol gets made in the first place. Once enough cortisol is circulating, it signals back to the hypothalamus and pituitary to ease off. That’s a built-in feedback loop, and it keeps the system from running indefinitely.

How is the HPA axis different from fight-or-flight?

The instant fight-or-flight reaction to danger, a racing heart and a rush of adrenaline, comes from a separate, faster pathway: the sympathetic-adrenal-medullary (SAM) system. A 2018 review in Frontiers in Behavioral Neuroscience describes the SAM system firing within seconds. The HPA axis activates just after, and it sustains the response over minutes to hours. Your amygdala typically flags the threat that sets both systems moving, but the HPA axis is built for the longer stretch, not the first spike.

What happens when the HPA axis stays switched on?

The HPA axis is meant to power down once a stressor passes, not stay lit indefinitely. Neuroscientist Bruce McEwen’s concept of “allostatic load” describes what happens when stress systems like this one activate too often, or for too long, without real recovery. The result is a kind of cumulative wear that can show up as disrupted sleep, a foggy mind, or feeling permanently on edge. Our hyperarousal entry covers what that feels like day to day, and our guide to stress and burnout goes deeper on what helps.

Where does bilateral stimulation fit in?

The HPA axis is one piece of the same stress-response system behind a tight chest or a racing mind. Most bilateral-stimulation research measures calming shifts in heart-rate variability and breathing rather than cortisol directly, but it targets that same broader system. Bilateral stimulation is the left-right rhythm at the center of EMDR. EmEase, a self-guided EMDR app, offers a guided version of this practice for everyday stress at app.emease.com.

Frequently asked questions

What is the HPA axis in simple terms?

It's the hormone relay your body uses to manage stress: your hypothalamus signals your pituitary gland, which signals your adrenal glands to release cortisol. It's slower than the instant fight-or-flight jolt of adrenaline, built to sustain a response rather than spark it, and a feedback loop normally shuts it back down once the stressor passes.

What are the three parts of the HPA axis?

The hypothalamus, a brain structure that senses the demand and releases CRH; the pituitary gland, which responds by releasing ACTH; and the adrenal glands, which respond to ACTH by releasing cortisol into your bloodstream. Each hormone triggers the next step in sequence, over roughly seconds to minutes.

What's the difference between the HPA axis and fight-or-flight?

'Fight-or-flight' often refers to the fast sympathetic-adrenal-medullary system, which releases adrenaline within seconds. The HPA axis is slower, taking over to sustain the response with cortisol over minutes to hours. Both usually start when your amygdala flags a threat, but they run on very different timelines.

What happens when the HPA axis becomes dysregulated?

When stress hits too often, or for too long, without real recovery, this system can stay activated past the point it's useful. Neuroscientist Bruce McEwen's 'allostatic load' model describes the resulting wear on the body, which can show up as disrupted sleep, a foggy mind, or feeling constantly on edge.

Does bilateral stimulation affect the HPA axis?

That specific link hasn't been well studied yet. Bilateral stimulation research mostly measures calming shifts in heart-rate variability and breathing, not cortisol directly. Since both are part of the same broader stress-response system, a connection is plausible, but it isn't something current research has confirmed.

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