The HPA axis controls the stress response: corticotropin-releasing hormone (CRH) from the hypothalamus stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), which stimulates cortisol release from the adrenal cortex. Cortisol provides negative feedback, shutting down the axis. Chronic stress dysregulates this system, elevating baseline cortisol and impairing feedback, contributing to depression, anxiety, and cognitive impairment.
The HPA axis is the body's main hormonal stress response pathway — a three-stage amplification chain from brain to blood. From your work on the anterior pituitary and hypothalamic hormone axes, you know how hypothalamic releasing hormones trigger pituitary responses. The HPA axis works the same way: the hypothalamus detects stress and releases corticotropin-releasing hormone (CRH) into the portal blood connecting it to the anterior pituitary. The pituitary responds by releasing adrenocorticotropic hormone (ACTH) into systemic circulation. ACTH travels to the adrenal cortex — the outer layer of the adrenal glands sitting atop the kidneys — and triggers release of cortisol, a steroid hormone with widespread metabolic and immunological effects.
Cortisol serves the body well in short bursts. It mobilizes energy (raising blood glucose), suppresses immune activity, sharpens attention, and prepares the organism to handle a threat. The critical elegance of the system is its negative feedback loop: cortisol itself circulates back to the hypothalamus and pituitary, binding receptors there that inhibit further CRH and ACTH release. Once the stressor is resolved, cortisol shuts off its own production — a self-terminating response. This is homeostatic regulation in action: the product of the cascade feeds back to suppress the cascade.
The problem arises with chronic stress. When stressors are prolonged — persistent work pressure, trauma, poverty — the HPA axis is activated repeatedly or continuously. Over time, cortisol receptors in the hypothalamus and hippocampus become downregulated (reduced in number or sensitivity), degrading the feedback signal. The axis loses its ability to self-regulate: baseline cortisol rises, remains elevated even after stressors pass, and the shutdown mechanism becomes sluggish. This is HPA dysregulation — the system stuck in an "on" state.
Chronically elevated cortisol has cascading consequences. It damages hippocampal neurons (which are dense with glucocorticoid receptors), impairing memory consolidation and further weakening HPA feedback — a vicious cycle. It chronically suppresses immune function, increasing vulnerability to infection. And it dysregulates mood circuits: elevated cortisol is strongly associated with major depression, particularly melancholic presentations with early morning awakening and flattened diurnal cortisol rhythms. Understanding the HPA axis provides the biological bridge between life stress, brain structure, and mood disorder — a foundation for everything from antidepressant mechanisms to the neurobiology of trauma.