ACTH Blood Test Explained: What It Measures and Why It's Ordered

What ACTH Is and Where It Comes From

Adrenocorticotropic hormone (ACTH, also called corticotropin) is a 39-amino-acid peptide produced by corticotroph cells of the anterior pituitary gland, where it is cleaved from the larger precursor pro-opiomelanocortin (POMC). Hypothalamic corticotropin-releasing hormone (CRH) stimulates its release; circulating cortisol suppresses it through negative feedback. ACTH in turn acts on the adrenal cortex to drive cortisol synthesis, and that feedback loop is the reason the hormone is measured at all. Reviews of the diagnostic workup of cortisol excess have described this hypothalamic–pituitary–adrenal axis as the organising framework for interpreting ACTH results alongside cortisol rather than in isolation (PMID 35979805). Secretion is pulsatile and follows a diurnal pattern, which is one reason published diagnostic protocols specify when blood is drawn (PMID 16378774).

What the ACTH Blood Test Actually Measures

The test quantifies immunoreactive ACTH in a blood sample. A review of the analytical and clinical aspects of adrenocorticotrophin determination described the shift from older radioimmunoassays to two-site immunometric (sandwich) assays and discussed how different antibody pairs recognise the intact molecule and its fragments to differing degrees, so that results are method-dependent (PMID 14503983). That same review emphasised that ACTH is a labile peptide and that pre-analytical handling — collection tube, temperature and time to separation — materially affects the number a laboratory reports (PMID 14503983).

Plasma Rather Than Serum

Most published protocols call for plasma collected into EDTA, kept cold and separated quickly, rather than serum, because of proteolytic degradation and adsorption of the peptide; the analytical review discussed these handling requirements as central to obtaining a usable ACTH value (PMID 14503983). Results may be reported in pg/mL or pmol/L depending on the laboratory, and reference intervals are assay- and time-of-day-specific, which is why published guidance frames interpretation around the laboratory's own method rather than a universal cut-off (PMID 14503983).

Why Clinicians Order an ACTH Measurement

In the published literature, a plasma ACTH level is rarely a first-line screening test. It is described instead as a classification step performed after a biochemical abnormality in cortisol has already been established.

Separating ACTH-Dependent From ACTH-Independent Cortisol Excess

A 2022 review of the diagnostic workup of Cushing's syndrome described a sequential approach in which screening tests — such as late-night salivary cortisol, low-dose dexamethasone suppression and 24-hour urinary free cortisol — establish hypercortisolism, after which plasma ACTH is used to distinguish ACTH-dependent from ACTH-independent disease and to direct imaging and, where indicated, inferior petrosal sinus sampling (PMID 35979805). An earlier clinical review of the evaluation and treatment of Cushing's syndrome described the same logic: confirm the excess first, then use ACTH to localise the driver before committing to surgery (PMID 16378774). Researchers evaluating the national diagnostic criteria for Cushing's disease in Japan analysed how hormonal criteria, including dynamic endocrine testing, performed in classifying patients (PMID 23171704).

Separating Primary From Secondary Adrenal Insufficiency

When cortisol is deficient, the direction of the ACTH value is the discriminator described in the literature. A review of corticotroph deficiency described secondary (pituitary) adrenal insufficiency as a state in which ACTH is inappropriately low or normal despite low cortisol, and discussed its causes including pituitary disease and exogenous glucocorticoid exposure (PMID 22516763). In primary adrenal failure, by contrast, loss of cortisol feedback allows ACTH to rise; a report of long-lasting subclinical Addison's disease described a prolonged compensated phase in which pituitary drive was increased before overt clinical insufficiency developed (PMID 17853338). The same principle is used in veterinary medicine: a review of canine hypoadrenocorticism described endogenous ACTH measurement as the means of separating primary from secondary disease once adrenal insufficiency had been confirmed (PMID 25813848).

Looking for a Non-Pituitary Source

A Japanese review of ectopic ACTH syndrome described tumours outside the pituitary — including small-cell lung carcinoma and bronchial carcinoid tumours — that secrete ACTH autonomously, and discussed the difficulty of localising the source in some patients (PMID 15148825). The 2022 workup review likewise described the separation of pituitary from ectopic ACTH secretion as one of the harder problems in the field, addressed with dynamic testing and venous sampling rather than a single ACTH number (PMID 35979805).

What Published Studies Associate With High ACTH Values

Higher ACTH concentrations are described in the literature in two broad situations. The first is loss of cortisol negative feedback: in primary adrenal insufficiency, pituitary output rises, and a published account of subclinical Addison's disease described this compensated pattern persisting over a long period before clinical decompensation (PMID 17853338). The veterinary review of canine hypoadrenocorticism described the same physiology in dogs, where elevated endogenous ACTH indicated adrenal rather than pituitary failure (PMID 25813848).

The second is autonomous ACTH production. In ACTH-dependent Cushing's syndrome, reviews described detectable or frankly elevated ACTH in the face of cortisol excess as the signal pointing away from an adrenal tumour and toward a pituitary corticotroph adenoma or an ectopic source (PMID 16378774). The ectopic ACTH syndrome review described neuroendocrine and lung tumours as recognised causes of this pattern (PMID 15148825). None of these reviews described a single threshold value as diagnostic on its own; each placed the value within a staged workup (PMID 35979805).

What Published Studies Associate With Low or Suppressed ACTH Values

Low ACTH alongside low cortisol was described in the corticotroph deficiency review as the hallmark of secondary adrenal insufficiency, including the suppression that follows exogenous glucocorticoid exposure and pituitary disease or surgery (PMID 22516763). Low or undetectable ACTH alongside high cortisol has a different meaning: reviews of Cushing's syndrome described suppressed ACTH as the finding that identifies ACTH-independent, adrenal-driven hypercortisolism and redirects imaging to the adrenal glands (PMID 16378774, PMID 35979805). The paired cortisol value, not the ACTH number alone, carries the interpretation in both scenarios.

An ACTH Level and an "ACTH Stimulation Test" Are Not the Same Thing

The phrase "ACTH test" is used in the literature for two distinct procedures. A plasma ACTH measurement quantifies the patient's own hormone. An ACTH stimulation test administers synthetic ACTH (cosyntropin/tetracosactide) and measures the cortisol response to assess adrenal reserve. A review of pitfalls in interpreting the cosyntropin stimulation test described how factors such as the timing of testing, recent pituitary injury, altered cortisol-binding proteins and assay differences can produce misleading results in the diagnosis of adrenal insufficiency (PMID 30855285). The canine review described the ACTH stimulation test as the central confirmatory test for hypoadrenocorticism in dogs, with endogenous ACTH reserved for classification afterwards (PMID 25813848).

Stimulation testing has also been studied outside the cortisol axis. A 2024 cross-sectional study examined the plasma aldosterone response to an ACTH stimulation test as a diagnostic approach in primary aldosteronism, and researchers reported on its performance relative to conventional confirmatory testing in that cohort (PMID 38481234).

Assay, Sampling and Interpretation Caveats: What Studies Report

Published reviews repeatedly return to the point that an ACTH result is only as good as the sample and the assay behind it.

• Peptide instability. The analytical review reported that ACTH degrades readily and that collection, cooling and prompt separation are critical to a valid result (PMID 14503983).
• Method dependence. The same review described differences between immunoassay designs in how they recognise intact ACTH versus precursor and fragment forms, so values are not freely interchangeable between laboratories (PMID 14503983).
• Pseudo-Cushing states and confounders. A review of pitfalls in the diagnosis of hypercortisolaemia described conditions, medications and physiological states that distort cortisol axis testing and can mimic or mask true disease (PMID 26608494).
• Fluctuating and mild disease. A review of pitfalls in diagnosing Cushing's syndrome described cyclic and mild forms in which single time-point testing can be normal, so repeat testing was discussed as part of the diagnostic strategy (PMID 18209858).
• Stimulation-test misreads. The cosyntropin pitfalls review reported that normal responses can occur early after pituitary injury and that assay and binding-protein differences affect cortisol thresholds (PMID 30855285).
• Criteria performance. Researchers evaluating Japan's diagnostic criteria for Cushing's disease analysed how well the defined hormonal criteria classified patients, illustrating that diagnostic rules themselves are studied and revised (PMID 23171704).

How ACTH Sits Within a Wider Workup

What a Single ACTH Result Cannot Settle

Across the sources above, no review described a standalone ACTH value as diagnostic. The published workups pair it with cortisol, consider the time of day, account for medications and intercurrent illness, and repeat or extend testing when the clinical picture and the laboratory disagree (PMID 35979805, PMID 18209858). Reference intervals differ by assay, by collection protocol and by laboratory, and the analytical review reported that these method differences are substantial enough that results should be read against the issuing laboratory's own ranges (PMID 14503983).

This page is for educational purposes only and is not medical advice; consult a licensed physician or the clinician who ordered the test about what any individual ACTH result means, whether repeat or confirmatory testing is appropriate, and how it fits the rest of the clinical picture. Nothing here is a reference range, a threshold or an interpretation of a specific report.

References

• Diagnostic workup of Cushing's syndrome (Journal of Neuroendocrinology, 2022)
• Evaluation and treatment of Cushing's syndrome (The American Journal of Medicine, 2005)
• Analytical and clinical aspects of adrenocorticotrophin determination (Annals of Clinical Biochemistry, 2003)
• Corticotroph deficiency (Annales d'Endocrinologie, 2012)
• [Ectopic ACTH syndrome] (Nihon Rinsho, 2004)
• Long-lasting subclinical Addison's disease (Experimental and Clinical Endocrinology & Diabetes, 2007)
• Canine hypoadrenocorticism: pathogenesis, diagnosis, and treatment (Topics in Companion Animal Medicine, 2014)
• Pitfalls in the interpretation of the cosyntropin stimulation test for the diagnosis of adrenal insufficiency (Current Opinion in Endocrinology, Diabetes, and Obesity, 2019)
• [Pitfalls in the diagnosis of hypercortisolemia] (Polski Merkuriusz Lekarski, 2015)
• Pitfalls in the diagnosis of Cushing's syndrome (Arquivos Brasileiros de Endocrinologia e Metabologia, 2007)
• Evaluation of the diagnostic criteria for Cushing's disease in Japan (Endocrine Journal, 2013)
• Plasma aldosterone response to ACTH stimulation test for diagnosis of primary aldosteronism: a cross-sectional study (BMC Endocrine Disorders, 2024)