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HPA Axis
The hypothalamic-pituitary-adrenal axis is a major neuroendocrine regulatory system connecting the hypothalamus, anterior pituitary, and adrenal cortex. Through sequential secretion of CRH, ACTH, and cortisol, the HPA axis regulates the physiological stress response, metabolism, immune activity, cardiovascular function, and circadian patterns of cortisol secretion.
RegionHead and Neck
SystemEndocrine System
The hypothalamic-pituitary-adrenal axis, commonly called the HPA axis, is a major neuroendocrine regulatory system linking the hypothalamus, anterior pituitary gland, and adrenal cortex. It coordinates hormonal responses to physiological and psychological stress while also contributing to normal regulation of metabolism, immune activity, cardiovascular function, and circadian physiology.
The core pathway consists of three major hormonal signals. The hypothalamus releases corticotropin-releasing hormone (CRH), CRH stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), and ACTH stimulates the adrenal cortex to produce cortisol.
Cortisol then acts throughout the body and provides negative feedback to both the hypothalamus and anterior pituitary. This feedback system allows cortisol secretion to increase when required while limiting excessive or prolonged activation.
Components of the HPA Axis
The HPA axis consists of three principal anatomical components:
- Hypothalamus
- Anterior pituitary gland
- Adrenal cortex
These structures communicate through hormonal signals carried in the circulation.
Overview of the HPA Axis
| Level | Structure | Major Hormone | Primary Target |
|---|---|---|---|
| 1 | Hypothalamus | CRH | Anterior pituitary |
| 2 | Anterior pituitary | ACTH | Adrenal cortex |
| 3 | Adrenal cortex | Cortisol | Multiple tissues throughout the body |
Hypothalamus
The hypothalamus forms the highest endocrine regulatory level of the HPA axis. It integrates neural, hormonal, circadian, and stress-related information and converts these inputs into neuroendocrine signals.
For the HPA axis, the most important hypothalamic neurons are located within the paraventricular nucleus.
Paraventricular Nucleus
The paraventricular nucleus (PVN) is located within the hypothalamus near the third ventricle. It contains several populations of neurosecretory neurons.
Parvocellular neurosecretory neurons involved in HPA regulation synthesize and release CRH into the hypothalamic-hypophyseal portal circulation.
Corticotropin-Releasing Hormone
Corticotropin-releasing hormone (CRH) is the principal hypothalamic hormone regulating ACTH secretion.
When the HPA axis is activated, CRH is released from hypothalamic neurosecretory terminals in the median eminence.
Median Eminence
The median eminence is a specialized region at the base of the hypothalamus where hypothalamic releasing hormones enter the primary capillary plexus of the hypophyseal portal system.
CRH released here is transported directly to the anterior pituitary without first entering the general systemic circulation.
Hypothalamic-Hypophyseal Portal System
The hypothalamic-hypophyseal portal system provides the vascular connection between the hypothalamus and anterior pituitary.
This arrangement allows small quantities of hypothalamic releasing hormones to reach anterior pituitary cells at relatively high local concentrations.
Portal Transport of CRH
CRH enters capillaries in the median eminence and travels through hypophyseal portal veins to the secondary capillary plexus within the anterior pituitary.
It then diffuses from the circulation and binds receptors on corticotroph cells.
Anterior Pituitary
The anterior pituitary, or adenohypophysis, forms the second major level of the HPA axis.
Its corticotroph cells respond to hypothalamic CRH by synthesizing and releasing ACTH.
Corticotrophs
Corticotrophs are endocrine cells of the anterior pituitary that synthesize the precursor protein proopiomelanocortin (POMC).
Proteolytic processing of POMC produces ACTH and other related peptides.
Proopiomelanocortin
Proopiomelanocortin (POMC) is a large precursor polypeptide expressed in corticotroph cells and several other tissues.
Within anterior pituitary corticotrophs, POMC processing produces ACTH as a major biologically active product.
Adrenocorticotropic Hormone
Adrenocorticotropic hormone (ACTH), also called corticotropin, is released from the anterior pituitary into the systemic circulation.
ACTH travels to the adrenal glands and acts primarily on steroidogenic cells of the adrenal cortex.
Adrenal Glands
The paired adrenal glands are located superior and slightly medial to the kidneys in the retroperitoneum.
Each gland consists of an outer adrenal cortex and inner adrenal medulla. The HPA axis primarily regulates the adrenal cortex.
Adrenal Cortex
The adrenal cortex contains three major zones:
- Zona glomerulosa
- Zona fasciculata
- Zona reticularis
ACTH has its most important acute effects on the zona fasciculata and zona reticularis.
Zona Fasciculata
The zona fasciculata is the thick middle layer of the adrenal cortex and is the principal site of cortisol synthesis.
Its steroidogenic cells are arranged predominantly in cords separated by sinusoidal capillaries.
Zona Reticularis
The zona reticularis is the innermost cortical layer. It produces adrenal androgens and also contributes to glucocorticoid production.
ACTH is an important regulator of steroidogenesis within this region.
Zona Glomerulosa
The zona glomerulosa produces aldosterone. Although ACTH can have transient effects on aldosterone secretion, the principal regulators of the zona glomerulosa are the renin-angiotensin system and extracellular potassium concentration.
For this reason, aldosterone regulation is not considered the primary output of the HPA axis.
ACTH Receptors
ACTH acts on adrenal cortical cells through the melanocortin 2 receptor (MC2R).
Activation of MC2R stimulates intracellular signaling pathways that promote cholesterol availability and steroid hormone synthesis.
Cholesterol and Cortisol Synthesis
Like other steroid hormones, cortisol is synthesized from cholesterol.
ACTH promotes movement of cholesterol into mitochondria, an important early step in adrenal steroidogenesis.
StAR Protein
The steroidogenic acute regulatory protein (StAR) facilitates transport of cholesterol to the inner mitochondrial membrane, where steroid synthesis begins.
ACTH stimulates processes that increase StAR activity and steroidogenic capacity.
Cortisol
Cortisol is the principal glucocorticoid in humans and represents the major hormonal output of the HPA axis.
It is a steroid hormone and therefore can cross cell membranes and bind intracellular glucocorticoid receptors.
Cortisol Transport
Most circulating cortisol is bound to plasma proteins, particularly corticosteroid-binding globulin and albumin.
A smaller unbound fraction is biologically available to enter target cells and interact with glucocorticoid receptors.
Glucocorticoid Receptor
The glucocorticoid receptor is an intracellular receptor expressed in many tissues.
After cortisol binds the receptor, the hormone-receptor complex can influence gene transcription and alter cellular function.
Major Actions of Cortisol
Cortisol has widespread physiological effects. Major actions include:
- Supporting glucose availability
- Modifying protein and lipid metabolism
- Supporting cardiovascular responsiveness
- Modulating immune and inflammatory activity
- Participating in physiological adaptation to stress
- Influencing central nervous system function
Metabolic Effects
Cortisol helps maintain adequate energy substrate availability during fasting and stress.
It promotes hepatic gluconeogenic processes and influences protein and lipid metabolism to provide substrates that can support energy production.
Cardiovascular Effects
Normal glucocorticoid concentrations help maintain vascular responsiveness to catecholamines and contribute to normal blood pressure regulation.
Severe cortisol deficiency can therefore impair cardiovascular stability, particularly during physiological stress.
Immune and Inflammatory Effects
Cortisol influences numerous components of immune and inflammatory signaling.
At elevated concentrations, glucocorticoids suppress many inflammatory and immune responses, a property that is also used therapeutically with synthetic glucocorticoid medications.
Stress Response
The HPA axis is activated by many forms of physiological stress. Examples can include illness, injury, hypoglycemia, major surgery, and other conditions that threaten homeostasis.
Neural pathways convey information about these challenges to hypothalamic regulatory centers, increasing CRH secretion and subsequent ACTH and cortisol release.
Sequence of HPA Axis Activation
| Step | Event |
|---|---|
| 1 | Stress-related signals activate hypothalamic regulatory pathways |
| 2 | Paraventricular neurons release CRH |
| 3 | CRH reaches anterior pituitary corticotrophs through portal blood |
| 4 | Corticotrophs release ACTH |
| 5 | ACTH reaches the adrenal cortex through systemic circulation |
| 6 | Adrenal cortical cells increase cortisol synthesis and secretion |
| 7 | Cortisol acts on peripheral tissues and feeds back to the hypothalamus and pituitary |
Negative Feedback
Negative feedback is fundamental to regulation of the HPA axis.
As circulating cortisol rises, it inhibits further activation of upstream components of the axis.
Feedback at the Hypothalamus
Cortisol acts on the hypothalamus and associated neural circuits to reduce drive for CRH secretion.
This decreases stimulation of anterior pituitary corticotrophs.
Feedback at the Anterior Pituitary
Cortisol also acts directly on anterior pituitary corticotrophs to suppress ACTH synthesis and secretion.
This reduces stimulation of the adrenal cortex and limits further cortisol production.
Feedback Loop
| Hormone | Source | Action Within Axis |
|---|---|---|
| CRH | Hypothalamus | Stimulates ACTH secretion |
| ACTH | Anterior pituitary | Stimulates adrenal cortisol production |
| Cortisol | Adrenal cortex | Inhibits hypothalamic and pituitary activation |
Long-Loop Feedback
Feedback produced by a peripheral endocrine hormone acting on upstream regulatory centers is commonly called long-loop feedback.
Cortisol inhibition of both the pituitary and hypothalamus is a classic example.
Circadian Regulation
The HPA axis is not activated only during stress. It also follows a strong circadian rhythm under normal conditions.
Cortisol concentrations vary predictably across the day in relation to the sleep-wake cycle and central circadian regulation.
Suprachiasmatic Nucleus
The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the principal central circadian pacemaker.
It receives information about environmental light through pathways originating in the retina and influences neuroendocrine rhythms, including activity of the HPA axis.
Daily Cortisol Pattern
Under a typical sleep-wake schedule, cortisol concentrations rise during the latter portion of sleep and are generally highest around the early morning and awakening period.
Levels then tend to decline over the course of the day and reach relatively low concentrations during the late evening and early sleep period.
Cortisol Awakening Response
A further increase in cortisol frequently occurs shortly after awakening. This phenomenon is called the cortisol awakening response.
It represents a distinct feature of normal daily HPA-axis regulation.
Pulsatile Secretion
ACTH and cortisol are secreted in pulses rather than at a completely constant rate.
These ultradian fluctuations are superimposed on the broader circadian rhythm of the HPA axis.
CRH and Vasopressin
CRH is the principal hypothalamic regulator of ACTH, but vasopressin can also contribute to corticotroph activation.
During stress, CRH and vasopressin can act together to enhance ACTH secretion.
HPA Axis and Sympathetic Response
The HPA axis is one component of the integrated response to stress. Another major component involves activation of the sympathetic nervous system and adrenal medulla.
These systems interact but have different anatomical pathways and hormonal outputs.
HPA Axis Versus Sympathoadrenal System
| Feature | HPA Axis | Sympathoadrenal System |
|---|---|---|
| Central regulator | Hypothalamus | Central autonomic pathways |
| Pituitary involvement | Yes | No |
| Adrenal region | Cortex | Medulla |
| Major adrenal output | Cortisol | Epinephrine and norepinephrine |
| Hormone class | Steroid | Catecholamine |
ACTH and Adrenal Trophic Effects
ACTH does more than acutely stimulate cortisol secretion. It also has trophic effects on the adrenal cortex.
Persistent ACTH stimulation can promote growth of ACTH-responsive cortical zones, while prolonged ACTH suppression can lead to cortical atrophy.
Exogenous Glucocorticoids
Glucocorticoid medications can reproduce cortisol-mediated negative feedback on the hypothalamus and pituitary.
Prolonged exposure to sufficiently high doses can suppress CRH and ACTH secretion and reduce endogenous adrenal cortisol production.
HPA Axis Suppression
When ACTH remains suppressed for an extended period, ACTH-dependent portions of the adrenal cortex can become atrophic.
For this reason, abrupt withdrawal after prolonged systemic glucocorticoid exposure can be clinically important because endogenous cortisol production may not immediately recover.
Primary Adrenal Insufficiency
Primary adrenal insufficiency results from dysfunction of the adrenal cortex itself.
Reduced cortisol production decreases negative feedback, leading to increased CRH and ACTH secretion when the hypothalamus and pituitary remain functional.
HPA Pattern in Primary Adrenal Insufficiency
| Hormone | Typical Direction |
|---|---|
| Cortisol | Decreased |
| ACTH | Increased |
Secondary Adrenal Insufficiency
Secondary adrenal insufficiency results from inadequate pituitary ACTH secretion.
Without sufficient ACTH stimulation, cortisol production by the adrenal cortex decreases.
HPA Pattern in Secondary Adrenal Insufficiency
| Hormone | Typical Direction |
|---|---|
| ACTH | Low or inappropriately normal |
| Cortisol | Decreased |
Tertiary Adrenal Insufficiency
Tertiary adrenal insufficiency refers to deficient hypothalamic stimulation of the pituitary-adrenal system, resulting in inadequate ACTH and cortisol secretion.
Suppression following prolonged exposure to exogenous glucocorticoids is an important setting in which central HPA-axis suppression can occur.
Primary Versus Central Adrenal Insufficiency
| Feature | Primary | Central |
|---|---|---|
| Primary defect | Adrenal cortex | Pituitary or hypothalamic regulation |
| Cortisol | Low | Low |
| ACTH | Usually high | Low or inappropriately normal |
| Aldosterone deficiency | Can be prominent | Usually less affected because regulation is primarily through renin-angiotensin and potassium |
Cushing Syndrome
Cushing syndrome refers to the clinical state produced by chronic excessive glucocorticoid exposure.
Excess cortisol can arise from several different mechanisms, and the ACTH concentration helps distinguish ACTH-dependent from ACTH-independent causes.
ACTH-Dependent Hypercortisolism
In ACTH-dependent forms of hypercortisolism, excessive ACTH stimulates the adrenal cortex and increases cortisol production.
Sources can include a pituitary corticotroph tumor or ectopic ACTH production.
Cushing Disease
Cushing disease specifically refers to hypercortisolism caused by an ACTH-secreting pituitary corticotroph adenoma.
The term should be distinguished from Cushing syndrome, which includes hypercortisolism from any cause.
ACTH-Independent Hypercortisolism
When cortisol is produced autonomously by an adrenal lesion, elevated cortisol suppresses pituitary ACTH through negative feedback.
ACTH is therefore generally reduced in endogenous ACTH-independent adrenal hypercortisolism.
Hormonal Patterns in HPA Disorders
| Condition | ACTH | Cortisol |
|---|---|---|
| Primary adrenal insufficiency | High | Low |
| Secondary adrenal insufficiency | Low or inappropriately normal | Low |
| Pituitary ACTH-dependent Cushing disease | Inappropriately normal or high | High |
| Autonomous adrenal cortisol excess | Low | High |
Evaluation of the HPA Axis
Assessment of HPA-axis function can involve measurement of basal hormone concentrations and dynamic endocrine testing.
The appropriate test depends on whether the clinical question concerns cortisol deficiency, cortisol excess, pituitary ACTH secretion, or adrenal responsiveness.
Morning Cortisol
Because cortisol secretion follows a circadian rhythm, the timing of serum cortisol measurement is important.
Morning measurements are commonly used when evaluating suspected adrenal insufficiency because cortisol is normally relatively high during this period.
ACTH Measurement
Measurement of plasma ACTH together with cortisol helps identify the anatomical level of HPA-axis dysfunction.
For example, low cortisol with markedly elevated ACTH suggests a primary adrenal defect, while low cortisol with low or inappropriately normal ACTH suggests central dysfunction.
ACTH Stimulation Test
The ACTH stimulation test evaluates the ability of the adrenal cortex to produce cortisol in response to ACTH receptor stimulation.
A synthetic ACTH analogue is administered and cortisol is measured before and after stimulation.
Dexamethasone Suppression Testing
Dexamethasone suppression testing uses the negative-feedback properties of a potent synthetic glucocorticoid to evaluate regulation of cortisol secretion.
It is commonly used as part of the investigation of suspected endogenous hypercortisolism.
CRH Testing
Administration of CRH can be used in selected clinical settings to evaluate ACTH responses and help characterize abnormalities within the HPA axis.
Interpretation depends on the clinical context and the specific testing protocol.
Urinary Free Cortisol
Measurement of urinary free cortisol over a defined collection period can provide information about integrated cortisol production and is used in the evaluation of suspected cortisol excess.
Late-Night Salivary Cortisol
Normal circadian regulation produces low cortisol concentrations late at night.
Loss of this normal nadir can occur in hypercortisolism, making late-night salivary cortisol useful in appropriate diagnostic settings.
HPA Axis and Homeostasis
The HPA axis helps preserve internal stability when physiological demands change.
Its actions coordinate energy availability, cardiovascular responsiveness, inflammatory regulation, and adaptation to stress while negative feedback limits unnecessary hormonal exposure.
Integration With Other Endocrine Systems
Cortisol can influence other endocrine pathways, including thyroid, growth, reproductive, and metabolic systems.
Conversely, nutritional state, sleep, circadian signals, illness, and other hormonal systems can modify HPA-axis activity.
HPA Axis Regulation at a Glance
| Signal | Source | Target | Effect |
|---|---|---|---|
| CRH | Hypothalamus | Anterior pituitary | Increases ACTH release |
| ACTH | Anterior pituitary | Adrenal cortex | Increases cortisol synthesis |
| Cortisol | Adrenal cortex | Multiple tissues | Coordinates glucocorticoid responses |
| Cortisol feedback | Adrenal cortex | Hypothalamus and pituitary | Reduces further HPA-axis activation |
Key Anatomical Relationships
| Structure | Important Relationship |
|---|---|
| Paraventricular nucleus | Major hypothalamic source of CRH |
| Median eminence | Site where CRH enters portal circulation |
| Hypophyseal portal vessels | Carry CRH to the anterior pituitary |
| Anterior pituitary corticotrophs | Produce ACTH from POMC |
| Adrenal zona fasciculata | Principal site of cortisol production |
| Suprachiasmatic nucleus | Contributes to circadian regulation of the axis |
Key Features of the HPA Axis
| Feature | Key Point |
|---|---|
| Full name | Hypothalamic-pituitary-adrenal axis |
| Hypothalamic hormone | CRH |
| Pituitary hormone | ACTH |
| Principal adrenal hormone | Cortisol |
| Primary adrenal target | Zona fasciculata |
| Feedback mechanism | Cortisol inhibits hypothalamic and pituitary activation |
| Major hypothalamic nucleus | Paraventricular nucleus |
| Pituitary cell | Corticotroph |
| ACTH precursor | POMC |
| Major physiological roles | Stress adaptation, metabolism, cardiovascular support and immune modulation |
| Normal temporal pattern | Circadian and pulsatile secretion |
Anatomical and Physiological Importance
The HPA axis demonstrates how anatomically separated endocrine structures function as a single regulatory network. Neurosecretory cells within the hypothalamus detect and integrate information from the nervous system, circadian pathways, and the internal physiological environment. They translate these signals into CRH secretion at the median eminence.
The hypothalamic-hypophyseal portal circulation then carries CRH directly to the anterior pituitary, where corticotrophs release ACTH. ACTH enters the systemic circulation and reaches the adrenal cortex, particularly the zona fasciculata, stimulating production of cortisol from cholesterol.
Cortisol has effects throughout the body, but it also returns regulatory information to the structures that initiated its secretion. By suppressing hypothalamic and pituitary activity, cortisol completes a negative-feedback loop that stabilizes the system and limits excessive glucocorticoid exposure.
The HPA axis also illustrates the importance of temporal organization in endocrine physiology. Cortisol secretion varies with circadian timing and occurs in pulses, while acute physiological stress can temporarily increase activity above the normal daily pattern. The resulting hormonal output therefore reflects both baseline biological rhythms and changing physiological demands.
Because the axis contains several anatomically distinct levels, characteristic hormonal patterns can help localize endocrine dysfunction. Abnormalities can originate in the hypothalamus, pituitary gland, adrenal cortex, or from external glucocorticoid exposure. Understanding the normal pathway from CRH to ACTH to cortisol, together with cortisol-mediated negative feedback, provides the anatomical and physiological framework for interpreting these disorders.
Published on September 30, 2026
Last updated on September 30, 2026
Last updated on September 30, 2026