The Hypothalamic-Pituitary-Adrenal (HPA) Axis: The Body's Stress Circuit and Its Diagnostic Peptides

The hypothalamic-pituitary-adrenal (HPA) axis is the body's central stress-response circuit — a three-tier chain of command that controls the production of cortisol, the hormone that helps us mobilize energy, maintain blood pressure, and respond to physical and emotional stress. It is also a beautiful example of how peptides at each level can be turned into precise diagnostic tools.

The three tiers

The axis is a cascade, each level releasing a signal that acts on the next:

1. Hypothalamus → CRH. The hypothalamus releases corticotropin-releasing hormone (CRH), a 41-amino-acid peptide, into a private blood supply that feeds the pituitary.
2. Pituitary → ACTH. CRH tells the pituitary's corticotroph cells to release adrenocorticotropic hormone (ACTH), a 39-amino-acid peptide, into the bloodstream.
3. Adrenal glands → cortisol. ACTH travels to the adrenal cortex and stimulates it to synthesize and release cortisol (and adrenal androgens).

Cortisol then feeds back to the hypothalamus and pituitary to turn the signal down — a classic negative-feedback loop that keeps levels in range. Cortisol also follows a daily rhythm, peaking in the early morning and falling overnight.

Probing the axis with peptides

Because each tier uses a peptide signal, clinicians can inject a synthetic version of that signal and watch the response — a way to test exactly which level is working or failing.

Corticorelin (synthetic CRH) — tests the top two tiers

Corticorelin is synthetic CRH. Injected, it stimulates the pituitary to release ACTH, and then the adrenals to release cortisol. Its main use is the CRH stimulation test in Cushing's syndrome (cortisol excess): pituitary tumors causing Cushing's disease usually still respond to CRH by raising ACTH and cortisol, whereas ectopic ACTH-producing tumors and adrenal tumors usually do not — helping localize where the excess comes from.

Cosyntropin (synthetic ACTH 1-24) — tests the bottom tier

Cosyntropin is the active 1-24 fragment of ACTH. It skips the pituitary and directly tests the adrenal glands. In the ACTH stimulation test for adrenal insufficiency, cortisol is measured before and after a dose: a healthy adrenal raises cortisol above a threshold, while a blunted response indicates insufficiency.

| Peptide test | What is injected | What it tests |

| --- | --- | --- |

| CRH stimulation (corticorelin) | CRH | Pituitary ACTH response + adrenal response |

| ACTH stimulation (cosyntropin) | ACTH 1-24 | Adrenal cortisol response directly |

Reading the whole axis

Combining these dynamic peptide tests with a simple baseline ACTH measurement lets endocrinologists map the problem. High ACTH with low cortisol points to a failing adrenal (primary insufficiency, Addison's disease). Low ACTH with low cortisol points to a pituitary or hypothalamic cause (secondary/tertiary). High cortisol that stays high despite feedback points to Cushing's syndrome — and the CRH test then helps find the source.

When treatment enters the picture

Diagnosis and treatment are separate. Adrenal insufficiency is treated by replacing the missing hormones (hydrocortisone, and a mineralocorticoid in primary disease), not with these peptides. For pituitary Cushing's disease, surgery is first-line, and the somatostatin-analog peptide pasireotide is one medical option that suppresses ACTH from the tumor. The diagnostic peptides — corticorelin and cosyntropin — are probes, not therapies.

The HPA axis shows how understanding a signaling chain, tier by tier, turns each peptide messenger into a tool for reading the system's health. Compare this with the reproductive axis in GnRH agonists vs antagonists.

> This article is educational and not medical advice. HPA-axis testing is performed and interpreted by clinicians.