HPA Axis Function and Regulation

Overview of HPA Axis

The HPA axis is a neuroendocrine system that mediates the body's stress response through a cascade of hormonal signals originating in the hypothalamus, transmitted through the pituitary gland, and culminating in cortisol release from the adrenal glands, as described by the Endocrine Society 1, 2
The HPA axis represents one of the main components of the stress system and is essential for maintaining physiological homeostasis in response to internal or external stimuli, according to the American Physiological Society 1, 2

The hypothalamus initiates the cascade by releasing corticotropin-releasing factor (CRF), also called corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary gland to secrete adrenocorticotropin hormone (ACTH) into the circulation, as outlined by the European Society of Endocrinology 1, 2, 3
The pituitary amplifies the signal by secreting ACTH, which acts on the adrenal cortex to produce cortisol, a crucial hormone for maintaining homeostasis, as noted by the American College of Endocrinology 1, 2, 3

The HPA axis functions as the final common pathway in the stress response, with activation proportional to the intensity of emotional and systemic stressors, as described by the BioScience Trends journal 3
The HPA axis coordinates with the immune system, and abnormal functioning induces release of multiple hormones, including cortisol, pro-inflammatory cytokines, and glucocorticoids, which can contribute to depression and anxiety disorders, as reported by the BioScience Trends journal 3

Chronic stress leads to persistent HPA axis activation, resulting in insulin resistance, reduced hippocampal and medial prefrontal cortex volume, and decreased hippocampal neurogenesis, as noted by the Praxis Medical Insights journal 4
Altered HPA axis activity is one of the most commonly observed neuroendocrine abnormalities in patients with depressive disorder, according to the BioScience Trends journal 3
Hyperthyroidism can precipitate anxiety through cross-communication between the central thyroid system and HPA-related noradrenergic and serotonergic pathways, as described by the Praxis Medical Insights journal 5

The 1 mg dexamethasone suppression test is used to screen for autonomous cortisol secretion, as recommended by the Praxis Medical Insights journal 6
Plasma free metanephrines or 24-hour urinary metanephrines are used to diagnose suspected pheochromocytoma, as noted by the Praxis Medical Insights journal 6

CRH increases exponentially during stress conditions and reaches levels observed only under stress in the median eminence connecting the hypothalamus with the pituitary, according to the American Psychological Association 7
Mental stress induces higher levels of adrenocorticotropin hormone (ACTH) and profound enhancement of stress responses through CRH-mediated pathways, as reported by the BioScience community 8
Androgens, such as testosterone, are not involved in the primary stress response pathway, as stated by the American College of Clinical Psychology 7, 9

Excessive excitation of the hypothalamic‑pituitary‑adrenal (HPA) axis—not reduced excitatory neurotransmission—is the primary mechanism driving elevated cortisol levels in generalized anxiety disorder, indicating that the disorder is characterized by heightened stress‑pathway activity. 10

Therapeutic strategies should aim to normalize HPA‑axis function; selective serotonin reuptake inhibitors (SSRIs) have been shown to reduce cortisol concentrations in individuals with generalized anxiety disorder, providing pharmacologic evidence that targeting HPA‑axis hyperactivity improves both hormonal and anxiety outcomes. 10