HPAC stands for the hypothalamic-pituitary-adrenocortical axis, a communication chain between three organs that controls your body’s stress response. You may also see it written as the HPA axis, which is the more common abbreviation. Both refer to the same system: the hypothalamus in your brain, the pituitary gland just below it, and the adrenal glands on top of your kidneys, all working together to produce cortisol and other stress hormones.
How the HPAC Axis Works
The HPAC axis operates like a relay. When your brain perceives a threat or stressor, it triggers a chain reaction of hormones that prepares your body to respond. The sequence unfolds in three steps:
- Step 1: Your hypothalamus releases a signaling hormone called CRH (corticotropin-releasing hormone).
- Step 2: CRH travels to the anterior pituitary gland, which responds by releasing ACTH (adrenocorticotropic hormone) into your bloodstream.
- Step 3: ACTH reaches your adrenal glands, specifically the outer layer called the adrenal cortex, and stimulates them to produce cortisol.
Cortisol is the final product of this chain. It’s a glucocorticoid hormone with receptors in nearly every tissue in your body, which is why it affects so many different functions: energy, metabolism, blood pressure, immune response, and cognition. Once cortisol levels rise high enough, they signal back to the hypothalamus to stop releasing CRH. This negative feedback loop is what shuts the stress response off once it’s no longer needed.
What Cortisol Does in Your Body
Cortisol is often called the “stress hormone,” but it does far more than manage stress. It helps regulate blood sugar, influences how your body stores and uses fat, and plays a central role in controlling inflammation. One of its most important jobs is modulating the immune system. Cortisol dampens the production of inflammatory signals, promotes the clearance of pathogens and cellular debris, and shifts immune cells toward a less reactive state. This is why corticosteroid medications, which mimic cortisol, are widely used to treat inflammatory and autoimmune conditions.
Cortisol levels naturally follow a daily rhythm. They peak in the early morning, typically between 6 a.m. and 8 a.m. (around 10 to 20 mcg/dL in blood tests), then gradually decline throughout the day, dropping to their lowest point around midnight (roughly 3 to 10 mcg/dL by 4 p.m.). This pattern is one reason you tend to feel most alert in the morning and wind down in the evening.
What Happens When the Axis Is Overactive
When the HPAC axis produces too much cortisol over a prolonged period, it can lead to a condition called Cushing’s syndrome. This can happen because of a tumor on the pituitary gland that keeps pumping out ACTH, a tumor on the adrenal gland itself, or, most commonly, from taking corticosteroid medications for a long time. The effects are wide-ranging and serious: weight gain concentrated in the face and midsection, high blood pressure, elevated blood sugar that can progress to diabetes, thinning skin, bone loss, and mood disturbances.
Even without a formal Cushing’s diagnosis, chronic stress can keep the axis running hotter than it should. Over time, sustained exposure to elevated cortisol is associated with osteoporosis, metabolic dysfunction, high cholesterol, and even neurodegeneration. The immune system also takes a hit. While short bursts of cortisol help coordinate your immune response, prolonged elevation suppresses the production of T cells and shifts your body away from the type of immune activity that fights infections and monitors for abnormal cells.
What Happens When the Axis Is Underactive
The opposite problem, too little cortisol, is called adrenal insufficiency. In its primary form (historically known as Addison’s disease), the adrenal glands themselves are damaged, often by an autoimmune process where the body’s own antibodies attack the adrenal cortex. In secondary adrenal insufficiency, the problem sits higher up the chain: the pituitary gland fails to produce enough ACTH, so the adrenals never get the signal to make cortisol.
One common way secondary insufficiency develops is when someone stops taking corticosteroid medications abruptly after using them for a long time. The external supply of cortisol suppresses the HPAC axis, causing the adrenal glands to shrink from disuse. If the medication is withdrawn too quickly, the glands can’t ramp back up fast enough. This is why tapering off steroids gradually is standard practice. Untreated adrenal insufficiency can be fatal, as cortisol is essential for maintaining blood pressure and responding to physical stress like illness or injury.
Common symptoms of low cortisol include persistent fatigue, muscle weakness, low blood pressure, salt cravings, and unintentional weight loss.
How Doctors Test HPAC Function
Testing the HPAC axis involves checking cortisol levels directly and, when needed, using provocation tests that challenge the system to see how it responds. Initial screening typically includes one or more of the following: collecting urine over 24 hours to measure total cortisol output, testing saliva late at night (when cortisol should be at its lowest), or using a dexamethasone suppression test. Dexamethasone is a synthetic cortisol that, in a healthy system, should tell the brain to stop making its own cortisol. If cortisol levels stay high despite that signal, it suggests the feedback loop is broken.
For suspected adrenal insufficiency, doctors may use an ACTH stimulation test. A synthetic form of ACTH is injected, and blood cortisol is measured afterward. If the adrenal glands don’t respond with a healthy rise in cortisol, it confirms they aren’t functioning properly. More specialized tests, like the insulin tolerance test or CRH stimulation test, can help pinpoint whether the problem originates in the hypothalamus, the pituitary, or the adrenal glands themselves. Blood tests for antibodies against a specific adrenal enzyme (21-hydroxylase) can also help detect autoimmune adrenal destruction in its early stages.
Supporting Healthy HPAC Function
Because the HPAC axis is fundamentally a stress-response system, the most direct way to influence it is by managing stress itself. Chronic psychological stress keeps the axis activated in the same way a physical threat would, and over time the feedback loop can become less efficient at shutting cortisol production down. Fatigue, insomnia, mood disturbances, and poor stress tolerance are among the most commonly overlooked symptoms of a dysregulated axis.
Sleep is one of the most powerful regulators. Cortisol’s daily rhythm is closely tied to your sleep-wake cycle, and disrupted or insufficient sleep can flatten or shift that rhythm in ways that leave you feeling wired at night and exhausted in the morning. Consistent sleep and wake times help reinforce the natural cortisol curve. Regular physical activity, stress-reduction practices, and addressing underlying sources of chronic psychological strain all contribute to keeping the axis responsive rather than stuck in overdrive.