ACTH Unit Converter
ACTH Unit Converter
Convert ACTH between pg/mL, ng/L and pmol/L. pg/mL and ng/L are the same number — the harder problem is that ACTH decays in a warm tube, so a low result may be the specimen rather than the patient.
ACTH converter
Mass ⇄ molarACTH 25 pg/mL from a specimen drawn at 08:30
The three units
pmol/L = pg/mL × 0.220211
derived from a molecular weight of 4,541.1 Da for ACTH(1–39)
- pg/mL = ng/L
- a picogram per millilitre and a nanogram per litre are the same concentration. Both are in routine use and neither changes the number, so a report in one can be read straight against an interval quoted in the other
- × 0.220211
- the molar factor, from the mass of full-length ACTH(1–39), 4,541.1 daltons. Most European laboratories report pmol/L and most North American ones pg/mL, so this is a conversion readers genuinely need rather than a table curiosity
- the rough shortcut
- dividing pg/mL by 4.5 gets within about 1% of the correct pmol/L, because 1/0.220211 is 4.541. Useful at the bedside; use the exact factor when the figure is going into a record
- ACTH(1–39), not the precursor
- the assay is directed at the intact 39-residue peptide cleaved from pro-opiomelanocortin. Some ectopic ACTH-secreting tumours release precursors and fragments that different assays detect to different degrees, which is one reason ACTH results from different platforms are not always interchangeable
- pre-analytical, not arithmetic
- the largest errors on this page are not unit errors. ACTH adsorbs to glass and is degraded by plasma peptidases; a specimen left at room temperature, collected into the wrong tube or separated late falls, and the result looks like pituitary suppression
Worked example
ACTH 25 pg/mL from a specimen drawn at 08:30
25 pg/mL = 25 ng/L — the same number in the other mass convention
25 × 0.220211 = 5.51 pmol/L, which is how most European laboratories would report it
The quick check: 25 ÷ 4.5 = 5.6, within a per cent of the exact answer
Against the quoted morning interval of 7.2–63.3 pg/mL (1.59–13.94 pmol/L), 25 pg/mL sits within it
And it still means nothing alone. The same 25 pg/mL alongside a frankly raised cortisol would be inappropriately high; alongside a cortisol of 80 nmol/L at 09:00 it would be inappropriately low. The pair is the result
The ACTH and cortisol pair — why neither is read alone
| ACTH | Cortisol | Usual interpretation |
|---|---|---|
| High | High | ACTH-dependent cortisol excess: pituitary corticotroph adenoma (Cushing’s disease) or an ectopic ACTH source |
| Low or undetectable | High | ACTH-independent cortisol excess: an adrenal adenoma or carcinoma — or exogenous glucocorticoid, which suppresses ACTH and is by far the commonest cause of a Cushingoid picture |
| High | Low | Primary adrenal insufficiency: the adrenal cannot respond, so ACTH rises. Usually with a raised renin and low aldosterone |
| Low or inappropriately normal | Low | Secondary adrenal insufficiency: pituitary or hypothalamic disease, or recent glucocorticoid withdrawal |
| Normal | Normal | Uninformative on its own. Both hormones are diurnal and a mid-range pair at an unrecorded time excludes very little |
Getting the specimen right — where most ACTH errors happen
| Step | Requirement | What goes wrong if it is missed |
|---|---|---|
| Timing | Draw between 07:00 and 10:00 for the reference interval quoted here, and record the actual time | ACTH peaks in the early morning and falls through the day; an afternoon sample read against a morning interval looks falsely low |
| Tube | EDTA (lavender top), plastic or siliconised glass | ACTH adsorbs to untreated glass; serum and heparin tubes are rejected by reference laboratories |
| Transport | Chill immediately in wet ice | Plasma peptidases degrade ACTH at ambient temperature; the result falls |
| Separation | Centrifuge immediately and transfer the plasma | Contact with cells continues the degradation |
| Storage | Freeze the plasma and keep it frozen until assay | Labcorp lists ACTH as unstable both at room temperature and refrigerated, and stable for 14 days only when frozen |
| Freeze–thaw | One cycle only | Repeat thawing degrades the peptide further |
A fragile peptide, a moving baseline, and a result that is half of a pair
Adrenocorticotropic hormone is the pituitary signal that drives cortisol production, a 39-residue peptide cleaved from pro-opiomelanocortin. The unit conversion is straightforward and worth stating plainly: picograms per millilitre and nanograms per litre are the same number, so no arithmetic is needed between them, and picomoles per litre — the convention in most European laboratories — is pg/mL multiplied by 0.220211, from a molecular mass of 4,541.1 daltons. Dividing by 4.5 is accurate to about one per cent if you need the answer in your head.
The difficulty with ACTH is not arithmetic. It is that the molecule is unstable in the tube. ACTH adsorbs to untreated glass and is broken down by peptidases in plasma, so a specimen that sat at room temperature, went into the wrong tube, or was separated late will read low — and it will read low in exactly the way that suggests a suppressed pituitary. The requirement is a chilled EDTA sample, placed on wet ice immediately, centrifuged without delay and the plasma frozen. Reference laboratories list ACTH as unstable both at room temperature and refrigerated. An unexpectedly low result should therefore raise a question about how the sample was handled before it raises one about the patient.
The second difficulty is timing. ACTH has a strong diurnal rhythm, peaking in the early morning and falling to a trough around midnight, and the reference interval quoted on this page belongs to a specimen drawn between seven and ten in the morning. A sample taken in the afternoon and read against a morning interval will look low when it is merely late. The collection time is part of the result and should be recorded with it.
Above all, an ACTH is half of a measurement. It is interpreted against a simultaneous cortisol, and the same ACTH value carries opposite meanings depending on what the cortisol is doing. A low ACTH with a high cortisol points to an adrenal source of cortisol excess, or far more often to glucocorticoid the patient is taking. A high ACTH with a high cortisol points to an ACTH-dependent cause — a pituitary corticotroph adenoma, or an ectopic source. A high ACTH with a low cortisol is primary adrenal insufficiency; a low or inappropriately normal ACTH with a low cortisol is secondary. Reported on its own, without a paired cortisol and without a time, an ACTH is very close to uninterpretable, however precisely its units have been converted.
Frequently asked questions
Is pg/mL the same as ng/L for ACTH?
Yes, exactly the same number. A picogram per millilitre and a nanogram per litre are the same concentration, so an ACTH of 25 pg/mL is 25 ng/L. The conversion that does change the number is to picomoles per litre: multiply by 0.220211, giving 5.51 pmol/L.
How do I convert ACTH from pg/mL to pmol/L?
Multiply by 0.220211, from a molecular weight of 4,541.1 daltons for ACTH(1–39). Dividing by 4.5 is a good enough approximation at the bedside, since 1 ÷ 0.220211 is 4.541. Most European reports use pmol/L and most North American ones pg/mL.
Why does ACTH have to be collected on ice?
Because it is degraded by plasma peptidases and adsorbs to glass, so a sample held at room temperature loses ACTH and reads spuriously low. The requirement is a plastic or siliconised EDTA tube, immediate chilling in wet ice, prompt centrifugation and frozen plasma. Reference laboratories list ACTH as unstable both at room temperature and refrigerated.
What time of day should ACTH be measured?
Between about seven and ten in the morning, which is when the published reference intervals apply, and the actual time should be recorded. ACTH is strongly diurnal, peaking in the early morning and reaching its trough near midnight, so an afternoon sample read against a morning interval looks falsely low.
What does a high ACTH with a high cortisol mean?
ACTH-dependent cortisol excess — either a pituitary corticotroph adenoma, which is Cushing’s disease, or an ectopic ACTH-secreting tumour. Contrast that with a low ACTH and a high cortisol, which points to an adrenal source or to glucocorticoid the patient is taking. The pair is what carries the meaning; neither hormone is interpreted alone.
Related calculators
References
- Labcorp. Test 004440: Adrenocorticotropic Hormone (ACTH), Plasma — reference interval 7.2–63.3 pg/mL for specimens collected 07:00–10:00; specimen stability and collection requirements. Accessed 2026.
- Nieman LK, Biller BMK, Findling JW, et al. The diagnosis of Cushing’s syndrome: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2008;93(5):1526–1540.
- Bornstein SR, Allolio B, Arlt W, et al. Diagnosis and treatment of primary adrenal insufficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2016;101(2):364–389.
- Evaluation of plasma ACTH stability using the Roche Elecsys immunoassay. Clin Chim Acta. 2020;503:15–21.
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.