Adrenocorticotropic hormone, usually called ACTH, is a 39-amino-acid peptide released by the pituitary gland that serves as the body’s primary signal telling the adrenal glands to produce cortisol. It sits at the center of one of the most important hormonal cascades in human physiology, linking the brain’s perception of stress, circadian rhythms, and immune challenges to the adrenal cortex’s steroid output. But ACTH does more than just relay a “make cortisol” message, and the ways it can malfunction, get measured, and even be used as a drug make it a surprisingly rich molecule to understand.
How ACTH Is Made
ACTH is not built from scratch as a standalone molecule. Instead, it is cut from a much larger precursor protein called pro-opiomelanocortin, or POMC. POMC is synthesized in specialized cells of the anterior pituitary gland called corticotrophs, and it contains the sequences for several different hormones packed into one chain. Enzymes called prohormone convertases (mainly PC1/3 and PC2) slice POMC at specific sites to release its component peptides, including ACTH, beta-endorphin, and alpha-MSH.1PubMed Central. 60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides The human pituitary uses both cathepsin L and prohormone convertase pathways to process POMC, with an enzyme called carboxypeptidase E performing additional trimming after the initial cuts.2PubMed Central. Human pituitary contains dual cathepsin L and prohormone convertase processing pathway components involved in converting POMC into the peptide hormones ACTH, alpha-MSH, and beta-endorphin
The fact that ACTH shares a precursor with beta-endorphin (the body’s own opioid-like molecule) and alpha-MSH (a melanocyte-stimulating hormone that darkens skin) has practical consequences. When something drives up POMC production, you don’t just get more ACTH; you get more of these sibling peptides too. That shared origin explains why certain ACTH-related diseases cause unexpected symptoms like skin darkening, a detail that becomes important when we look at conditions like Addison’s disease and ectopic ACTH syndrome.
The Daily Rhythm and the HPA Axis
ACTH release is not constant. It follows a pronounced daily cycle, peaking in the early morning hours and dropping to its lowest levels around midnight. This pattern is orchestrated by the hypothalamic-pituitary-adrenal (HPA) axis, a three-tier signaling chain. The hypothalamus produces corticotropin-releasing hormone (CRH), which travels a short distance to the anterior pituitary and tells corticotrophs to release ACTH. ACTH then travels through the bloodstream to the adrenal glands, where it triggers cortisol production. Cortisol, in turn, feeds back to the hypothalamus and pituitary to suppress further CRH and ACTH release, closing the loop.
Beneath the broad daily arc, ACTH comes out in pulses. Animal studies show that pituitary corticotrophs fire off bursts of ACTH roughly every few minutes, with a secondary, slower rhythm of about one episode every 100 minutes. CRH from the hypothalamus does not create these pulses from nothing; rather, it amplifies them. When researchers blocked CRH in rats, the fast underlying pulses continued at the same frequency but shrank in size, and the slower rhythm nearly vanished.3PubMed. Effects of immunoneutralization of corticotropin-releasing hormone on ultradian rhythms of plasma adrenocorticotropin In other words, the pituitary has its own intrinsic pulsatility, and CRH acts as a volume knob rather than an on/off switch.
The daily schedule is further shaped by the hypothalamus’s gene-transcription patterns. CRH gene activity in the hypothalamus follows its own clock, with a nocturnal surge that helps replenish the CRH stores needed for the next morning’s ACTH peak. Arginine vasopressin (AVP) plays a supporting role, though its gene activity in unstressed animals is barely detectable unless cortisol feedback is removed.4Endocrinology. Corticotropin-Releasing Hormone and Arginine Vasopressin Gene Transcription in the Hypothalamic Paraventricular Nucleus of Unstressed Rats This layered design, with an intrinsic rhythm amplified by CRH and fine-tuned by cortisol feedback, makes the system both robust and flexible.
Negative Feedback and Why It Matters
Cortisol’s ability to shut down its own production is one of the most clinically important features of the HPA axis. The mechanism works at multiple levels. At the pituitary, cortisol (and other glucocorticoids) activates a receptor called the glucocorticoid receptor (GR), which then binds to a region of the POMC gene’s promoter and directly suppresses ACTH synthesis. GR can also block the activity of Nur77, a transcription factor that would otherwise ramp up POMC expression. Back at the hypothalamus, cortisol inhibits both CRH production and secretion.5PubMed Central. Role of glucocorticoid negative feedback in the regulation of HPA axis pulsatility
This feedback loop is why people who take prescription glucocorticoids (like prednisone) for weeks or months cannot simply stop overnight. The external cortisol suppresses the HPA axis so thoroughly that their own ACTH production withers. Tapering gives the axis time to wake back up. It is also the principle behind diagnostic tests for cortisol overproduction: if you give a patient a synthetic glucocorticoid and their ACTH and cortisol fail to drop as expected, something in the system is ignoring the brake signal.
How ACTH Actually Triggers Cortisol
When ACTH reaches the adrenal cortex, it binds to the melanocortin-2 receptor (MC2R) on cells in the zona fasciculata, the middle layer of the adrenal cortex. MC2R is unusual among hormone receptors because it cannot reach the cell surface or respond to ACTH without an escort protein called MRAP (melanocortin-2 receptor accessory protein). Without MRAP, the receptor gets stuck inside the cell and never makes it to the outer membrane. People who are born lacking functional MRAP are resistant to ACTH and become glucocorticoid-deficient, a condition called familial glucocorticoid deficiency type 2.6PubMed Central. Structure and function of the melanocortin2 receptor accessory protein (MRAP) MRAP itself forms an unusual structure, pairing up in antiparallel homodimers, and research has confirmed that when it is present, MC2R reaches the cell surface, gets properly glycosylated, and signals normally in response to ACTH.7PubMed Central. Melanocortin-2 receptor accessory protein MRAP forms antiparallel homodimers
Once ACTH binds MC2R and triggers the cAMP-protein kinase A (PKA) signaling cascade, the next bottleneck is getting cholesterol, the raw material for all steroid hormones, into the mitochondria where the first enzymatic step occurs. This is handled by the Steroidogenic Acute Regulatory protein, or StAR. ACTH rapidly increases both the expression and activity of StAR, which shuttles cholesterol across the mitochondrial membranes.8PubMed Central. ACTH Action on StAR Biology This step is rate-limiting for steroid production, meaning that StAR activation is effectively the moment when ACTH’s signal becomes cortisol output. Studies in bovine adrenal cells have directly confirmed that ACTH activates StAR protein expression in these cortical cells.9PubMed. Regulation of expression of the steroidogenic acute regulatory (StAR) protein by ACTH in bovine adrenal fasciculata cells
ACTH and the Stress Response
Under acute stress, the HPA axis fires up fast. ACTH is among the earliest hormonal responders. A study measuring hormonal and immune responses to a standardized laboratory stressor found that noradrenaline and ACTH showed the fastest and strongest stress responses, followed by cortisol, the inflammatory marker IL-6, and leptin.10PubMed Central. The impact of acute stress on hormones and cytokines, and how their recovery is affected by music-evoked positive mood This makes sense given the signaling chain: ACTH rises within minutes because CRH triggers its release directly, while cortisol takes longer because the adrenal glands need time to synthesize it after receiving the ACTH signal.
The speed of the ACTH response is important clinically. In emergency situations, critically ill patients, and surgical settings, ACTH levels spike rapidly and can be used to gauge how well the HPA axis is functioning. A blunted ACTH response to severe illness sometimes points to underlying adrenal or pituitary problems that need attention.
Actions Beyond the Adrenal Glands
Although cortisol production is ACTH’s best-known job, the hormone has direct effects on other tissues. One well-studied example is fat metabolism. ACTH can stimulate lipolysis, the breakdown of stored fat, by acting directly on adipose tissue. In classic experiments, injecting ACTH into rats tripled the release of free fatty acids and more than doubled glycerol release from fat tissue, effects that occurred even in adrenalectomized animals that had no adrenal glands to produce cortisol.11PubMed. Effect of ACTH on lipolysis in adipose tissue of normal and adrenalectomized rats in vivo This confirms that at least some of ACTH’s metabolic effects are direct and not merely a consequence of the cortisol it stimulates.
ACTH also acts on melanocytes, the pigment-producing cells in skin. Because ACTH shares structural features with alpha-MSH (they both come from the same POMC precursor and ACTH’s first 13 amino acids are identical to alpha-MSH), high ACTH levels can stimulate melanocortin-1 receptors in the skin and cause hyperpigmentation. This is clinically visible in Addison’s disease, where the adrenal glands fail and ACTH levels soar in a futile attempt to drive cortisol production. Patients often develop a characteristic bronzing of the skin, particularly in skin creases, gums, and areas exposed to friction.
Additionally, immune cells can both produce and respond to ACTH-related peptides. Cytokines released during infection, such as IL-2, can stimulate ACTH secretion from the pituitary, creating a direct communication channel between the immune system and the stress-hormone axis.12PubMed. The mechanism of action of cytokines to control the release of hypothalamic and pituitary hormones in infection This immune-endocrine crosstalk helps explain why severe infections can disrupt cortisol regulation and why chronic inflammatory conditions sometimes affect HPA axis function.
When ACTH Goes Wrong
Disorders of ACTH fall into two broad categories: too much and too little.
Excess ACTH most commonly comes from a small, benign pituitary tumor (a corticotroph adenoma) that churns out ACTH regardless of cortisol feedback, leading to Cushing’s disease. Less commonly, tumors outside the pituitary can produce ACTH ectopically. Ectopic ACTH secretion accounts for roughly 5 to 15 percent of Cushing’s syndrome cases and is most often traced to neuroendocrine tumors, frequently in the lungs or pancreas.13PubMed Central. A rare case of ectopic ACTH syndrome caused by primary renal neuroendocrine tumor Locating the culprit tumor can be maddeningly difficult. One published case described a patient whose source of ectopic ACTH was not identified until 16 years after the initial Cushing’s diagnosis, when advanced imaging finally revealed a small pancreatic neuroendocrine tumor.14JCEM Case Reports. The Elusive Neuroendocrine Tumor: Finding the Ectopic ACTH Source 16 Years After the Diagnosis of Cushing Syndrome
On the deficiency side, Addison’s disease (primary adrenal insufficiency) destroys the adrenal cortex, usually through autoimmune attack, leaving the glands unable to respond to ACTH. The pituitary compensates by cranking out more ACTH, sometimes to extremely high levels, which produces the skin darkening noted earlier. Secondary adrenal insufficiency, by contrast, results from pituitary damage or prolonged glucocorticoid therapy that suppresses ACTH itself, leaving the adrenals intact but understimulated.
Congenital adrenal hyperplasia (CAH), most often caused by 21-hydroxylase deficiency, provides another instructive example. The enzymatic block prevents normal cortisol synthesis, which removes negative feedback and allows ACTH to climb. Chronically elevated ACTH then drives the adrenal glands to overproduce androgens (the steroid precursors that pile up before the enzymatic block), often accelerating skeletal maturation and causing premature closure of growth plates in children.15The Journal of Clinical Endocrinology & Metabolism. Genetics and Pathophysiology of Classic Congenital Adrenal Hyperplasia Due to 21-Hydroxylase Deficiency
Diagnosing ACTH-Related Disorders
Measuring ACTH in a clinical lab is straightforward in principle but trickier in practice. ACTH was long considered fragile in blood samples, requiring immediate chilling, rapid centrifugation, and frozen transport. More recent data paints a less dire picture. A systematic review found that ACTH remains stable in uncentrifuged EDTA tubes for about six hours at room temperature and at least eight hours if refrigerated.16PubMed Central. Is the stability of ACTH in whole blood a genuine concern during the preanalytical phase? A systematic review Similarly, evaluation of a widely used immunoassay found that a delay of up to four hours in separating plasma from cells did not meaningfully affect ACTH results, and the hormone remained within about 10 percent of baseline for up to 12 hours at room temperature.17PubMed. Evaluation of plasma ACTH stability using the Roche Elecsys immunoassay Extended storage beyond 12 hours, however, does cause degradation regardless of temperature. These findings are gradually loosening the strict specimen-handling protocols that some labs still enforce.
Distinguishing between pituitary and ectopic sources of ACTH excess remains a diagnostic challenge. The high-dose dexamethasone suppression test was traditionally used: in theory, a pituitary adenoma retains some sensitivity to glucocorticoid feedback and suppresses ACTH output in response to a large dexamethasone dose, while an ectopic tumor does not. In practice, the test’s real-world effectiveness has been questioned, with some researchers noting that its ability to discriminate between these sources is more limited than textbook descriptions suggest.18The Journal of Clinical Endocrinology & Metabolism. Effectiveness Versus Efficacy: The Limited Value in Clinical Practice of High Dose Dexamethasone Suppression Testing in the Differential Diagnosis of Adrenocorticotropin-Dependent Cushing’s Syndrome The CRH stimulation test is another tool: administering CRH and measuring the ACTH response can help, since pituitary tumors tend to respond to CRH while ectopic tumors often do not.19PubMed. The ovine corticotropin-releasing hormone stimulation test and the dexamethasone suppression test in the differential diagnosis of Cushing’s syndrome No single test is perfect, so clinicians typically combine biochemical testing with imaging studies, and sometimes inferior petrosal sinus sampling (drawing blood directly from the veins draining the pituitary), to pin down the source.
ACTH as a Drug
Synthetic and purified forms of ACTH have been used therapeutically for decades, sometimes in conditions that seem far removed from cortisol regulation. The best-established use is in infantile spasms (West syndrome), a severe form of epilepsy in infants. Controlled studies have shown that ACTH, particularly at higher doses, can rapidly and completely eliminate the characteristic seizure pattern and abnormal brain-wave activity. The mechanism appears to involve two pathways: ACTH stimulates the adrenals to release steroids that reduce brain inflammation, and it also acts directly on melanocortin receptors in the brain to dampen neuronal excitability.20PubMed Central. ACTH treatment of infantile spasms: mechanisms of its effects in modulation of neuronal excitability A comparison of high- and low-dose ACTH found similar seizure-control rates for both, but the high-dose regimen caused more side effects, including sleepiness and measurable brain shrinkage on imaging, suggesting that lower doses may offer a better risk-benefit balance in some patients.21PubMed. A comparative study of high-dose and low-dose ACTH therapy for West syndrome
ACTH preparations (marketed as repository corticotropin injection, or Acthar Gel in the U.S.) have also been used in certain kidney diseases. In membranous nephropathy, a condition where immune deposits damage the kidney’s filtration barrier and cause heavy protein loss in the urine, corticotropin injections have shown the ability to reduce proteinuria in patients who failed to respond to standard immunosuppressive drugs.22Kidney International Reports. Repository Corticotropin Injections Promote Clinical Remission and Regulatory T Cell Expansion in Membranous Nephropathy In one case report involving fibrillary glomerulonephritis, a rare and difficult-to-treat kidney disease, a patient achieved partial remission with corticotropin injections over several years, with proteinuria dropping from nephrotic-range levels to below one gram per day.23Clinical Kidney Journal. Treatment of fibrillary glomerulonephritis with use of repository corticotropin injections Whether ACTH’s benefit in these kidney conditions comes purely from adrenal cortisol production, from direct melanocortin receptor effects on immune cells, or from some combination remains an active question.
ACTH in Fetal Development
The fetal adrenal glands are remarkably active, and ACTH plays a central role in shaping their output. In primates, the fetal adrenal cortex is dominated by a transient structure called the fetal zone, which occupies roughly 80 to 90 percent of the cortical volume during midgestation. Under ACTH stimulation, this zone produces large quantities of DHEA-S, an androgenic steroid precursor that the placenta converts into estrogens needed to maintain pregnancy. The fetal adrenals also produce cortisol, which helps mature the lungs, liver, thyroid, and gut in preparation for life outside the womb.24Endocrine Reviews. Developmental and Functional Biology of the Primate Fetal Adrenal Cortex In some non-primate species, such as sheep, fetal cortisol also triggers labor, but this does not appear to be the case in humans, where the signals initiating parturition are more complex.
After birth, the fetal zone rapidly involutes and largely disappears within the first few months of life, replaced by the familiar adult adrenal cortex architecture. This dramatic remodeling is one of the most striking examples of how ACTH-driven adrenal function is tuned to the specific physiological needs of each life stage. Premature infants, whose fetal adrenal transition may not be complete, sometimes have difficulties with cortisol production that require medical support.