?An ACTH blood test measures a hormone produced by the pituitary gland that tells the adrenal glands to make cortisol.
That sounds straightforward. Interpretation is not.
ACTH changes naturally throughout the day, responds to stress and is usually most informative when measured at the same time as cortisol. A high or low ACTH result therefore cannot be interpreted sensibly without asking what the cortisol level was at the same time.
This relationship can help clinicians work out whether an abnormal cortisol result is more likely to originate in the adrenal glands themselves or higher up in the hormonal pathway, particularly the pituitary or hypothalamus.
There is another important distinction.
An ACTH blood test is not the same as an ACTH stimulation test or Short Synacthen Test.
A basal ACTH test measures the ACTH your own pituitary gland is producing.
A Synacthen test gives synthetic ACTH and then measures how strongly your adrenal glands respond by producing cortisol.
They answer different questions.
London Blood Tests offers a private ACTH Blood Test measuring adrenocorticotrophic hormone as a standalone biomarker, with clinic and home blood collection options.
What Is ACTH?
ACTH stands for adrenocorticotropic hormone, also written as adrenocorticotrophic hormone.
It is produced by specialised cells in the anterior pituitary gland.
The pituitary does not release ACTH randomly. It forms part of a tightly controlled hormonal system called the hypothalamic-pituitary-adrenal axis, or HPA axis.
The sequence works broadly like this:
The hypothalamus releases corticotrophin-releasing hormone.
That stimulates the pituitary gland to release ACTH.
ACTH travels through the bloodstream to the adrenal glands.
The adrenal cortex responds by producing cortisol.
Cortisol then feeds information back to the brain and pituitary, helping regulate further ACTH production.
This feedback system means ACTH and cortisol need to be understood together.
What Does an ACTH Blood Test Measure?
An ACTH blood test measures the concentration of naturally occurring ACTH in the blood.
You may also see it described as:
ACTH hormone blood test
ACTH lab test
adrenocorticotropic hormone test
adrenocorticotrophic hormone test
The test itself measures one hormone.
Its usefulness comes from comparing the ACTH result with cortisol and the clinical circumstances.
Gloucestershire Hospitals NHS guidance states that an isolated ACTH measurement is rarely diagnostically useful and should be interpreted with a concurrent cortisol result.
This is an important principle.
ACTH tells us about the signal being sent.
Cortisol tells us something about the adrenal response.
Looking at both helps locate where a disturbance may be occurring within the HPA axis.
ACTH and Cortisol: How Do They Work Together?
The relationship between ACTH and cortisol is based on feedback.
When cortisol is low, the pituitary would normally be expected to increase ACTH stimulation.
When cortisol is high, ACTH is normally suppressed.
That creates several potentially useful patterns.
High ACTH With Low Cortisol
This pattern can occur when the adrenal glands are not producing enough cortisol despite receiving a strong ACTH signal.
That can support concern about primary adrenal insufficiency, where the problem originates within the adrenal glands.
It does not identify the exact cause by itself.
Low or Inappropriately Normal ACTH With Low Cortisol
When cortisol is low but ACTH has not risen appropriately, the problem may be higher in the hormonal pathway.
Possible causes include pituitary or hypothalamic dysfunction, often described as secondary or central adrenal insufficiency.
Low ACTH With High Cortisol
If cortisol excess has already been established and ACTH is suppressed, this can suggest that cortisol production is occurring independently of pituitary ACTH stimulation.
Further specialist testing is needed to determine the source.
Normal or High ACTH With Confirmed High Cortisol
When cortisol excess has been established and ACTH is not suppressed, the excess may be ACTH-dependent.
Possible sources include pituitary ACTH production or ectopic ACTH production elsewhere in the body.
Again, an ACTH result does not determine the precise cause alone.
Endocrine investigations can involve suppression tests, stimulation tests and imaging depending on the suspected condition.
Is ACTH Usually Tested Alone?
Ideally, ACTH is interpreted with cortisol.
Several NHS laboratory services specifically request a simultaneous cortisol sample when ACTH is measured.
That is because a value that appears numerically "normal" may still be inappropriate for the cortisol concentration.
Imagine, for example, that cortisol is very low.
The physiologically appropriate response would normally be increased ACTH secretion.
An ACTH concentration technically sitting within a laboratory reference interval might therefore still be inappropriately normal in that context.
This is why endocrine interpretation cannot always be reduced to:
low,
normal,
high.
The relationship between hormones often matters more than a single flag beside one result.
London Blood Tests offers a separate Cortisol Blood Test, and our existing guide to cortisol testing, blood, saliva and 9am results explains why cortisol timing and sample type can alter interpretation.
Why Is an ACTH Blood Test Done in the Morning?
ACTH follows a circadian or diurnal rhythm.
Concentrations are normally higher earlier in the day and lower later.
This makes sample timing important.
UK laboratory guidance commonly recommends basal ACTH collection in the morning, often around 7am to 10am or 8am to 10am.
A random afternoon ACTH result should therefore not simply be compared with a morning reference interval.
Timing gives the result context.
The London Blood Tests ACTH page also recommends morning collection where possible.
For someone being investigated for adrenal insufficiency, NICE currently recommends an 8am to 9am serum cortisol as the initial cortisol assessment. ACTH may then provide additional information about whether reduced cortisol production appears primary or central.
Can Stress Affect ACTH Levels?
Yes.
ACTH forms part of the body's physiological stress response.
Physical and psychological stress can influence HPA-axis activity and may temporarily change ACTH secretion.
Even the stress associated with venepuncture has been recognised as a possible influence.
That does not make ACTH useless.
It means results need appropriate collection conditions and clinical context.
Someone who is acutely ill, severely sleep-deprived or under significant physiological stress may not produce the same ACTH result as they would under stable circumstances.
Where results are unexpected or borderline, repeat or more specific endocrine testing may sometimes be more useful than over-interpreting one isolated value.
Why ACTH Sample Handling Matters
ACTH is not quite like many routine blood biomarkers.
The hormone is relatively unstable after collection.
NHS laboratories commonly require ACTH blood to be collected into an EDTA tube and transported rapidly to the laboratory, sometimes chilled or on ice, with prompt separation and freezing of the plasma.
Oxford University Hospitals specifies EDTA plasma and requires the sample to be separated and frozen within six hours.
Other laboratories use even stricter transport requirements.
This is important for private testing because pre-analytical handling can matter just as much as taking the blood sample correctly.
ACTH is not an ideal test for a sample that is collected and then allowed to sit unrefrigerated for an extended period before processing.
Appropriate logistics should therefore be confirmed when arranging ACTH testing, particularly for home collection.
What Is the Difference Between an ACTH Blood Test and a Synacthen Test?
This is probably the most important distinction in the article.
Basal ACTH Blood Test
A basal ACTH blood test measures the ACTH naturally produced by your pituitary gland.
It usually involves a single timed blood sample, often taken together with cortisol.
It helps answer:
How much ACTH is the pituitary releasing?
Short Synacthen Test
A Short Synacthen Test is a dynamic adrenal function test.
Synacthen is a synthetic form of ACTH.
During the test, cortisol is measured before synthetic ACTH is administered. Synacthen is then given by injection and cortisol is measured again, commonly after 30 minutes and sometimes after 60 minutes depending on the protocol.
The question is:
Can the adrenal glands produce an adequate cortisol response when stimulated?
Therefore:
ACTH blood test = measures endogenous pituitary ACTH
Synacthen test = tests the adrenal response to synthetic ACTH
Confusing these tests can lead to completely incorrect interpretation.
Is an ACTH Blood Test the Main Test for Addison's Disease?
Not usually as the first step.
This is another area where private-testing websites can overstate the role of ACTH.
Addison's disease is a form of primary adrenal insufficiency.
Current NICE guidance recommends an 8am to 9am serum cortisol test when adrenal insufficiency is suspected.
ACTH becomes particularly useful when cortisol is low or otherwise concerning because the ACTH concentration can help distinguish primary adrenal failure from a pituitary or hypothalamic cause.
Some people then require a dynamic test such as a Short Synacthen Test.
So it is more accurate to say:
ACTH helps determine the likely level of dysfunction within the HPA axis.
It should not be presented as a standalone test that independently diagnoses Addison's disease.
If autoimmune primary adrenal disease is being investigated, London Blood Tests also offers an Adrenal Cortex Antibodies Test, which asks a different question: whether autoantibodies associated with autoimmune adrenalitis are detectable.
ACTH measures hormone signalling.
Adrenal cortex antibodies investigate a possible autoimmune cause.
They are not interchangeable.
What Is Primary Adrenal Insufficiency?
Primary adrenal insufficiency occurs when the adrenal cortex cannot produce sufficient adrenal hormones.
Because cortisol production falls, the pituitary attempts to compensate by releasing more ACTH.
The typical hormonal pattern can therefore involve:
low cortisol
with
high ACTH
One cause is autoimmune destruction of the adrenal cortex, often referred to clinically as Addison's disease.
Other causes exist.
An ACTH result alone cannot determine why the adrenal glands are failing.
Electrolytes, renin, aldosterone, adrenal antibodies and other investigations may be relevant depending on the clinical circumstances.
What Is Secondary Adrenal Insufficiency?
Secondary adrenal insufficiency occurs when ACTH stimulation from the pituitary is inadequate.
In this situation, the adrenal glands may be structurally capable of making cortisol but receive insufficient ACTH signalling.
The hormonal pattern can therefore involve:
low cortisol
with
ACTH that is low or not appropriately elevated
Pituitary disease, pituitary surgery and suppression of the HPA axis by glucocorticoid treatment are among the situations that can alter this pathway.
The distinction from primary adrenal insufficiency matters because the causes and associated hormonal abnormalities can differ.
Can Steroid Medication Affect ACTH and Cortisol?
Yes—substantially.
Glucocorticoid medicines can suppress the normal HPA axis.
This can occur with oral corticosteroids such as prednisolone or hydrocortisone, but clinically meaningful systemic exposure can also occur through other routes in some circumstances.
NICE specifically advises considering exogenous glucocorticoid exposure when interpreting morning cortisol results.
Do not stop prescribed steroids simply to obtain an ACTH or cortisol result.
Abrupt steroid withdrawal can be dangerous in someone whose HPA axis is suppressed.
If ACTH or adrenal-function testing is being arranged while you are using steroid treatment, follow the instructions of the healthcare professional or endocrinology team managing the investigation.
Dynamic tests such as the Short Synacthen Test have their own specific medication protocols.
What Does a High ACTH Result Mean?
A high ACTH result shows increased pituitary ACTH secretion at the time the sample was collected.
Its meaning depends on cortisol.
If cortisol is low, high ACTH can support primary adrenal insufficiency.
If cortisol excess has already been demonstrated, elevated or non-suppressed ACTH can support an ACTH-dependent cause of hypercortisolism.
Stress can also influence ACTH.
An isolated high result therefore does not automatically diagnose Addison's disease, Cushing's disease or an ACTH-producing tumour.
The next investigation depends on the cortisol pattern and clinical presentation.
What Does a Low ACTH Result Mean?
Low ACTH means pituitary ACTH secretion is reduced at the time of testing.
Again, cortisol determines how meaningful this is.
Low ACTH with low cortisol may support central adrenal insufficiency or HPA-axis suppression.
Low ACTH with confirmed cortisol excess may suggest ACTH-independent cortisol production.
But a single result still needs to be considered against:
sample timing,
medicines,
steroid exposure,
clinical symptoms,
and the cortisol concentration.
ACTH should not be interpreted by looking only at whether the laboratory has placed an arrow beside the number.
What Is a Normal ACTH Range?
There is no single reference interval that should be copied from one website and applied to every laboratory.
Different assays use different methods, units and reference ranges.
For example, Oxford University Hospitals currently lists an ACTH interval of 0–46 ng/L, while Newcastle Hospitals Laboratories lists 7.2–63.3 ng/L for samples collected between 7am and 10am.
That difference illustrates why the reference interval supplied by the laboratory that analysed your own specimen is the relevant one.
Timing also matters.
A morning reference range should not automatically be applied to a late-day sample.
Does an ACTH Blood Test Require Fasting?
Fasting is not usually the defining preparation requirement for a standalone basal ACTH measurement.
Timing and sample handling are generally more important.
Some laboratories may have their own preparation instructions, especially when ACTH is collected with cortisol or other endocrine markers.
Follow the instructions supplied for the actual test being performed.
If additional fasting biomarkers are being collected at the same appointment, fasting may be required because of those tests rather than ACTH itself.
Do not change prescribed steroid medication without specific medical advice.
ACTH and Cushing's Syndrome
Cushing's syndrome refers to the clinical state caused by excessive cortisol exposure.
ACTH testing does not usually establish cortisol excess as the first step.
Instead, ACTH becomes useful after hypercortisolism has been demonstrated, because it can help classify the condition as ACTH-dependent or ACTH-independent.
Suppressed ACTH can support an adrenal or exogenous source of cortisol excess.
Normal or raised ACTH in established hypercortisolism can point towards ACTH-dependent disease.
Further endocrine testing is then required to locate the source.
Royal Cornwall Hospitals currently describes ACTH as a second-line investigation in both Cushing's and Addison's pathways rather than the frontline screening test.
That is the more accurate way to position it.
ACTH vs Cortisol: Which Test Should You Have?
It depends on the clinical question.
If the question is simply whether morning cortisol production appears low, cortisol is often the more appropriate initial measurement.
If cortisol is abnormal and the clinician wants to determine where within the HPA axis the problem may originate, ACTH becomes much more informative.
A standalone Cortisol Blood Test is available through London Blood Tests.
The dedicated ACTH Blood Test measures ACTH itself.
In many endocrine investigations, however, ACTH and cortisol are best considered together rather than choosing one and ignoring the other.
For a more detailed explanation of cortisol timing and testing methods, see our Cortisol Test UK guide.
What About a Broader Pituitary Function Profile?
ACTH is produced by the pituitary, but it is only one part of pituitary function.
The pituitary also regulates thyroid, reproductive and growth-related hormone pathways.
London Blood Tests offers a Pituitary Function Profile containing cortisol, FSH, LH, prolactin, TSH and growth hormone.
Importantly, the current Pituitary Function Profile does not include ACTH.
This distinction matters.
Someone who specifically requires ACTH should not assume that purchasing a general pituitary panel automatically includes it.
Likewise, a standalone ACTH result cannot assess every pituitary hormone pathway.
The appropriate test depends on what endocrine question is being investigated.
What Happens After an Abnormal ACTH Result?
There is no single pathway for every abnormal result.
Low Cortisol With High ACTH
This can support concern about primary adrenal insufficiency.
Further endocrine assessment may include repeat or confirmatory cortisol assessment, a Short Synacthen Test, electrolytes, renin, aldosterone and adrenal antibodies depending on the situation.
Low Cortisol With Low or Inappropriately Normal ACTH
This pattern may raise concern about pituitary or hypothalamic dysfunction or HPA-axis suppression.
A broader pituitary assessment and endocrine review may be needed.
High Cortisol With Suppressed ACTH
If cortisol excess has already been confirmed, suppressed ACTH can suggest an ACTH-independent source.
Specialist investigation is required.
High Cortisol With Normal or High ACTH
Confirmed cortisol excess with non-suppressed ACTH suggests ACTH-dependent disease.
Further endocrine tests may then help distinguish pituitary from ectopic ACTH production.
Unexpected Isolated ACTH Result
Before drawing conclusions, check:
Was the sample collected at the correct time?
Was cortisol measured with it?
Was the specimen handled correctly?
Is the patient taking steroids or other relevant medicines?
Was there acute illness or major physiological stress?
Sometimes the most useful next step is not another large blood panel.
It is making sure the first result can actually be interpreted.
When Is Adrenal Insufficiency an Emergency?
Suspected adrenal crisis is a medical emergency.
Possible features can include severe weakness, vomiting, abdominal pain, very low blood pressure, collapse, confusion, severe dehydration or rapidly worsening illness in someone at risk of adrenal insufficiency.
If adrenal crisis is suspected, treatment should not be delayed while waiting for private ACTH testing.
Urgent medical assessment is required.
NICE specifically advises emergency management when adrenal insufficiency is suspected in an acutely unwell person.
Private blood testing is appropriate for planned investigation.
It is not a substitute for emergency treatment.
What Does the London Blood Tests ACTH Test Include?
The London Blood Tests ACTH Blood Test measures one biomarker:
ACTH (Adreno Corticotrophic Hormone)
The current listed price is £239, excluding the applicable sample-collection charge.
The current listed turnaround time is approximately 2 days after laboratory receipt.
Blood can be collected through an available clinic appointment or home/hotel phlebotomy.
Because ACTH is a time-sensitive and relatively unstable analyte, appropriate morning collection and prompt laboratory handling are particularly important.
Where ACTH is being investigated because of an abnormal cortisol result, simultaneous or appropriately timed cortisol testing may provide substantially more clinical information than ACTH alone.
Private ACTH Blood Testing in London
London Blood Tests provides private ACTH blood testing in London for people who have been advised to measure adrenocorticotropic hormone or who require further investigation of an abnormal cortisol result.
The standalone ACTH Blood Test measures ACTH directly.
The separate Cortisol Blood Test measures serum cortisol.
For broader pituitary-related hormone assessment, the Pituitary Function Profile measures several other pituitary-regulated hormones, although ACTH itself is not currently included.
Where autoimmune adrenal insufficiency is being investigated, the Adrenal Cortex Antibodies Test asks a different question by looking for evidence of autoimmune adrenal involvement.
Private testing can make laboratory access more convenient.
However, endocrine results involving ACTH and cortisol can be difficult to interpret correctly without clinical context, particularly when results are abnormal or steroid medication is involved.
Frequently Asked Questions About ACTH Blood Tests
What is an ACTH blood test?
An ACTH blood test measures adrenocorticotropic hormone produced by the pituitary gland. ACTH stimulates the adrenal glands to produce cortisol.
What does ACTH stand for?
ACTH stands for adrenocorticotropic hormone, also written as adrenocorticotrophic hormone.
Is ACTH the same as cortisol?
No. ACTH is produced mainly by the pituitary gland and stimulates the adrenal glands. Cortisol is produced by the adrenal cortex in response to hormonal signalling that includes ACTH.
Is ACTH normally tested with cortisol?
Often, yes. ACTH is much easier to interpret when a simultaneous or appropriately timed cortisol concentration is available.
Why is ACTH tested in the morning?
ACTH follows a daily rhythm and is normally higher earlier in the day. Morning sampling allows the result to be interpreted against appropriate timed reference intervals.
Does an ACTH blood test require fasting?
Usually not as a standalone requirement. Timing, medication and correct sample handling are generally more important.
Can stress affect ACTH?
Yes. Physiological and psychological stress can influence ACTH secretion.
What does high ACTH with low cortisol mean?
This pattern can support concern about primary adrenal insufficiency because the pituitary is producing more ACTH while the adrenal glands are not producing an adequate cortisol response.
What does low ACTH with low cortisol mean?
It may suggest central adrenal insufficiency involving inadequate pituitary or hypothalamic stimulation, although medication and other factors also need consideration.
Is an ACTH blood test the same as a Synacthen test?
No. A basal ACTH test measures your own ACTH. A Short Synacthen Test gives synthetic ACTH and measures the cortisol response from the adrenal glands.
Can an ACTH blood test diagnose Addison's disease?
ACTH can support differentiation of primary from central adrenal insufficiency, but it is not generally the frontline standalone test used to diagnose Addison's disease.
Can ACTH help investigate Cushing's syndrome?
Yes, but typically after cortisol excess has already been demonstrated. ACTH can help distinguish ACTH-dependent from ACTH-independent causes.
Can steroid medication affect ACTH?
Yes. Glucocorticoids can suppress the HPA axis and alter ACTH and cortisol results. Prescribed steroid treatment should not be stopped without medical advice.
Why does an ACTH sample need special handling?
ACTH is relatively unstable in blood after collection. Laboratories may require EDTA plasma, rapid transport, cooling and prompt processing or freezing.
What is an ACTH stimulation test?
An ACTH stimulation test, usually called a Short Synacthen Test in the UK, measures how well the adrenal glands produce cortisol after an injection of synthetic ACTH.
The Bottom Line
An ACTH blood test becomes most useful when it is treated as part of a hormonal relationship rather than an isolated number.
ACTH is the signal.
Cortisol is part of the adrenal response.
High ACTH with low cortisol can point towards a different part of the HPA axis from low ACTH with low cortisol.
Likewise, ACTH can help classify established cortisol excess, but it is usually not the frontline test used to establish Cushing's syndrome in the first place.
Timing matters.
Sample handling matters.
Steroid medication matters.
And an ACTH blood test should not be confused with an ACTH stimulation or Short Synacthen Test, which is a dynamic assessment of adrenal cortisol production after synthetic ACTH is administered.
For private testing in London, London Blood Tests offers a standalone ACTH Blood Test, a separate Cortisol Blood Test, a broader Pituitary Function Profile and Adrenal Cortex Antibodies testing where autoimmune adrenal disease is being investigated.
The objective is not simply to determine whether ACTH is "high" or "low".
It is to understand whether the ACTH result is physiologically appropriate for the cortisol level and the clinical situation.
Independent Medical Sources
Gloucestershire Hospitals NHS — ACTH
Oxford University Hospitals — Adrenocorticotrophic Hormone (ACTH)
NICE — Adrenal Insufficiency: Identification and Management
Newcastle Hospitals — ACTH, Plasma