ACTH-Dependent vs Independent Interpreter
ACTH-Dependent vs Independent Interpreter
Cushing’s syndrome has been confirmed; the question now is what is causing it, and plasma ACTH splits the differential in two. Read the next sentence before using this page: ACTH is not a screening test for Cushing’s syndrome and a single value is uninterpretable unless hypercortisolism has already been established on at least two first-line tests. Used as a screen it produces nonsense in both directions. For the screening step, see the 1 mg overnight dexamethasone suppression test interpreter, the late-night salivary cortisol interpreter and the urinary free cortisol unit converter.
What is driving the cortisol?
ACTH + paired cortisol → which branchA patient with established Cushing’s syndrome — an abnormal 1 mg overnight dexamethasone suppression test and two abnormal late-night salivary cortisols. A paired 09:00 sample gives cortisol 780 nmol/L and ACTH 15 pg/mL.
The thresholds, and the fact that they are not agreed
ADLM/AACC Clinical Laboratory News (2021) — ACTH-independent below 5 pg/mL; ACTH-dependent above 20 pg/mL; 5–20 pg/mL a grey zone commonly associated with ACTH-dependent causes.
Fleseriu 2021 consensus — prints no number at all. Its algorithm reads only "low ACTH" against "normal or high ACTH".
Nieman 2008 Endocrine Society guideline — does not address the differential diagnosis; it is a screening document.
- pg/mL = ng/L
- the same concentration written two ways, so a threshold quoted in one can be read directly against a result in the other. pmol/L is the other family: multiply pg/mL by 0.220211, or divide pmol/L by that. The five thresholds this page uses are 5, 10, 20 and 30 pg/mL, which are 1.1, 2.2, 4.4 and 6.6 pmol/L
- why five bands and not three
- because the two sources that print numbers disagree about the lower line (5 against 10 pg/mL) and the upper line is written as a range (20 to 30) rather than a number. Collapsing that into three bands would require choosing one source over the other silently, on a test that decides whether a patient has adrenal or pituitary surgery. The two extra bands hold the disagreement and say what it is
- after, not instead of
- ACTH does not diagnose Cushing’s syndrome and cannot exclude it. It is pulsatile, markedly diurnal, and in Cushing’s disease characteristically normal or only modestly raised — so it overlaps the reference interval comprehensively. The 2008 Endocrine Society guideline’s four first-line tests are urinary free cortisol, late-night salivary cortisol and the two dexamethasone protocols, and ACTH is not among them
- the paired sample
- ACTH and cortisol have to be drawn into the same venepuncture at the same moment, because it is the relationship between them that carries the information. The same principle governs the whole pathway: the 2021 consensus insists the patient be hypercortisolaemic at the time of inferior petrosal sinus sampling, for exactly this reason
- chilled tube, on ice, separated promptly
- ACTH is proteolytically degraded in whole blood at room temperature and adsorbs to glass. Every pre-analytical failure pushes the result DOWN, and a falsely low ACTH looks precisely like an adrenal cause. The ADLM review makes both points: the special conditions, and that plasma ACTH assays are neither standardised nor harmonised, so results are not always interchangeable between platforms
- ACTH(1-39), and its fragments
- the assays are directed at the intact 39-residue peptide. Some ectopic ACTH-secreting tumours release pro-opiomelanocortin precursors and fragments that different platforms detect to different degrees, which is another reason a single number on a single platform is not a diagnosis
Worked example
A patient with established Cushing's syndrome — an abnormal 1 mg overnight dexamethasone suppression test and two abnormal late-night salivary cortisols. A paired 09:00 sample gives cortisol 780 nmol/L and ACTH 15 pg/mL.
Hypercortisolism is established on two first-line tests, so the ACTH is being asked the right question at the right point in the sequence
Cortisol 780 nmol/L in the same sample is clearly raised, so the pair is interpretable
ACTH 15 pg/mL is above Endotext's 10 ng/L line for ACTH-independence and below the 20 pg/mL line for ACTH-dependence → the grey zone both sources describe
The ADLM review adds that values in 5 to 20 pg/mL are commonly associated with ACTH-dependent causes, and the mechanism agrees: truly autonomous adrenal cortisol should have suppressed the corticotrophs much harder than this
So: repeat the pair on a properly handled specimen, and move to a dynamic test — CRH or desmopressin — rather than deciding on the concentration
Had the same patient's ACTH been 3 pg/mL, the answer would have been ACTH-independent on both thresholds and the next step adrenal imaging. Had it been 45 pg/mL, ACTH-dependent on both, and the next step a pituitary MRI
The five bands, and which source supports each
| Plasma ACTH | pmol/L | Reading | Supported by |
|---|---|---|---|
| Below 5 pg/mL | Below 1.1 | ACTH-independent — adrenal | Both sources |
| 5 to 10 pg/mL | 1.1 to 2.2 | Probably ACTH-independent | Endotext only; ADLM calls it grey zone |
| 10 to 20 pg/mL | 2.2 to 4.4 | Indeterminate | Both — and both note it is often ACTH-dependent |
| 20 to 30 pg/mL | 4.4 to 6.6 | Probably ACTH-dependent | ADLM; inside Endotext’s 20–30 range |
| 30 pg/mL or above | 6.6 or above | ACTH-dependent — pituitary or ectopic | Both sources |
What each branch leads to
| ACTH-independent | ACTH-dependent | |
|---|---|---|
| Where the problem is | The adrenal gland itself | Pituitary corticotroph adenoma, or an ectopic ACTH source |
| Next imaging | Dedicated adrenal CT or MRI | Pituitary MRI; chest and abdomen if ectopic is suspected |
| Commonest cause | Unilateral cortisol-producing adenoma | Cushing’s disease — about 80% of endogenous ACTH-dependent cases |
| Other causes | Adrenocortical carcinoma; primary bilateral macronodular adrenal hyperplasia; primary pigmented nodular adrenocortical disease | Ectopic ACTH from a bronchial carcinoid, small cell lung cancer, thymic or pancreatic neuroendocrine tumour, or medullary thyroid carcinoma |
| Further tests | Usually none — imaging decides | CRH or desmopressin test, high-dose dexamethasone suppression, inferior petrosal sinus sampling |
| Classic clue to the alternative | A suppressed ACTH with a LOW measured cortisol is exogenous steroid, not an adrenal tumour | Severe hypokalaemic alkalosis, weight loss, pigmentation and rapid onset point away from the pituitary |
Why ACTH is a second-step test, and what the numbers do not settle
Cushing’s syndrome is diagnosed in two stages, and the stages ask different questions with different tests. The first asks whether there is pathological hypercortisolism at all, and the 2008 Endocrine Society guideline answers it with 24-hour urinary free cortisol, late-night salivary cortisol, the 1 mg overnight dexamethasone suppression test or the longer low-dose test, with a second abnormal result required before the diagnosis is accepted. The second asks what is driving it, and that is where plasma ACTH belongs. Sending ACTH during the first stage is the commonest way this test is misused: it is pulsatile, markedly diurnal, and in Cushing’s disease it is characteristically normal or only modestly raised, so a value inside the reference interval excludes nothing and a value above it proves nothing.
Used in the right place, the logic is simple and mechanistically clean. If the adrenal gland is producing cortisol autonomously, negative feedback should shut the pituitary corticotrophs down, and ACTH should be very low or undetectable. If cortisol is high because ACTH is high, the ACTH will be normal or raised — normal being entirely compatible with disease, since a normal ACTH in the face of a markedly raised cortisol is itself inappropriate. That is the whole test, and it is why the sample has to be paired: it is the relationship between the two hormones, not either number alone, that carries the information.
What the test does not do is produce an agreed number. The two sources that print thresholds disagree about the lower line — Endotext puts ACTH-independence below 10 ng/L, the ADLM/AACC review below 5 pg/mL — and both write the upper line as a range rather than a value. The 2021 Cushing’s disease consensus prints no number at all, reading only low against normal-or-high. That is not an oversight on anyone’s part. Plasma ACTH assays are neither standardised nor harmonised, so a threshold derived on one platform does not transfer cleanly to another, and the pre-analytical requirements are unforgiving: ACTH adsorbs to glass and is degraded by plasma peptidases, so it needs a chilled EDTA tube, ice, prompt separation and freezing. Every failure in that chain pushes the result down, and a falsely low ACTH is indistinguishable from an adrenal cause on paper.
Which is why the practical advice attached to every band on this page is the same: repeat the pair before acting on it, and treat an indeterminate value as a reason to move to a dynamic test rather than a reason to choose a side. The 1 mg overnight dexamethasone suppression test interpreter, the late-night salivary cortisol interpreter and the urinary free cortisol unit converter cover the screening step that has to come first; the adrenal incidentaloma interpreter covers the adrenal lesion the independent branch leads to; the ACTH unit converter and the cortisol unit converter convert the two results between units; and where the cortisol excess turns out to be iatrogenic, the glucocorticoid withdrawal interpreter covers stopping the drug.
Frequently asked questions
What ACTH level means the cause is adrenal rather than pituitary?
There is no single agreed number, and this page prints both published ones rather than choosing. Endotext’s Cushing’s syndrome chapter states that consistent ACTH measurements below 10 ng/L (2 pmol/L) essentially confirm ACTH-independent Cushing’s syndrome, and that levels consistently greater than 20 to 30 ng/L (4 to 6 pmol/L) make it ACTH-dependent. The ADLM/AACC review sets the independent line lower, below 5 pg/mL, the dependent line at above 20 pg/mL, and calls 5 to 20 pg/mL a grey zone commonly associated with ACTH-dependent causes. Note the word consistent in both: these are thresholds for repeated measurements on properly handled samples, not for one result.
Can ACTH be used to screen for Cushing’s syndrome?
No. It is a second-step test and only interpretable once hypercortisolism has been established. ACTH is pulsatile and strongly diurnal, and in Cushing’s disease — the commonest endogenous cause — it is characteristically normal or only modestly raised, so its reference interval overlaps the disease comprehensively. The 2008 Endocrine Society guideline’s first-line tests are 24-hour urinary free cortisol, late-night salivary cortisol, the 1 mg overnight dexamethasone suppression test and the 48-hour low-dose test, with a second abnormal test needed before the diagnosis stands. ACTH appears in none of them, and that absence is deliberate.
Why does ACTH need a chilled tube?
Because it is a 39-amino-acid peptide that is degraded by plasma peptidases in whole blood at room temperature and adsorbs to glass surfaces. The ADLM/AACC review states the requirement plainly: collection into a chilled tube and freezing immediately prior to analysis, because of rapid proteolytic degradation. The clinically important consequence is that every pre-analytical failure biases the result in the same direction — downwards — and a spuriously low ACTH beside a high cortisol looks exactly like an adrenal cause. If a result is unexpectedly low, ask how the specimen was handled before acting on it.
What does an indeterminate ACTH mean in practice?
That the concentration has not answered the question and a different kind of test has to. Both published sources treat roughly 5 to 20 pg/mL as a grey zone, and both note that results in it are more often ACTH-dependent than not — which fits the mechanism, since truly autonomous adrenal cortisol should have suppressed the corticotrophs much harder. Practically, three things follow: repeat the pair on a properly collected sample; move to a test that reads ACTH responsiveness rather than concentration, meaning the CRH or desmopressin test, and inferior petrosal sinus sampling where necessary; and image both the adrenals and the pituitary, because an incidental lesion at either site is common enough to mislead a decision made on one number.
Does a suppressed ACTH always mean an adrenal tumour?
No, and the commonest alternative is far commoner than the tumour. Exogenous glucocorticoid suppresses ACTH by design, and iatrogenic Cushing’s syndrome outnumbers every endogenous cause combined. Two patterns separate them. A patient on prednisolone or dexamethasone will have a suppressed ACTH with a low measured cortisol, because cortisol immunoassays do not detect those drugs — that is glucocorticoid-induced adrenal suppression, not an adrenal tumour. A patient on hydrocortisone will have a suppressed ACTH with a high measured cortisol, because hydrocortisone is cortisol, which mimics adrenal Cushing’s exactly. Every route counts, including inhaled, topical, intranasal and intra-articular. A mishandled specimen is the third alternative.
Related calculators
References
- Juszczak A, Morris D, Grossman A. Cushing’s syndrome. In: Feingold KR et al., eds. Endotext. South Dartmouth (MA): MDText.com; updated 5 September 2024. NBK279088. "Consistent ACTH measurements of <10 ng/L (2 pmol/L) essentially confirm ACTH-independent Cushing’s syndrome, and radiologic evaluation of adrenals is the next step in diagnosis. Conversely, if levels are consistently greater than 20-30 ng/L (4-6 pmol/L), Cushing’s syndrome is ACTH-dependent, due to pituitary disease or ectopic ACTH secretion."
- Plasma ACTH: tales of diagnostic misadventure and the path forward. Clinical Laboratory News, Association for Diagnostics & Laboratory Medicine (ADLM, formerly AACC); January 2021. "ACTH concentrations less than 5 pg/mL are considered consistent with ACTH-independent Cushing’s syndrome whereas values greater than 20 pg/mL are consistent with an ACTH-dependent cause." Also the source for the pre-analytical requirements and for the statement that plasma ACTH assays are neither standardized nor harmonized.
- Nieman LK, Biller BMK, Findling JW, Newell-Price J, Savage MO, Stewart PM, Montori VM. The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2008;93(5):1526–1540. doi:10.1210/jc.2008-0125. The four first-line tests, and the requirement for a second abnormal test. It does not address the differential diagnosis or print any ACTH threshold — which is why this page does not cite it for one.
- Fleseriu M, Auchus R, Bancos I, Ben-Shlomo A, Bertherat J, Biermasz NR, et al. Consensus on diagnosis and management of Cushing’s disease: a guideline update. Lancet Diabetes Endocrinol. 2021;9(12):847–875. doi:10.1016/S2213-8587(21)00235-7. Its algorithm measures ACTH only after hypercortisolism is documented and stratifies it as "low" against "normal or high" without a numeric cut-off. Source for "it is essential that the patient is hypercortisolaemic at the time of IPSS".
- data/_factors.json, analyte
acth: molecular weight 4,541.1 Da for ACTH(1-39), from UniProt P01189 residues 138–176. pmol/L = pg/mL × 0.220211, so pg/mL = pmol/L × 4.54109. The four thresholds on this page, 5, 10, 20 and 30 pg/mL, are 1.10, 2.20, 4.40 and 6.61 pmol/L.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.