Synacthen test: how the short ACTH stimulation test works and what the result means

The short Synacthen test checks whether your adrenal glands can make more cortisol when they are told to. You have a blood test, an injection of tetracosactide, a synthetic copy of the hormone ACTH sold as Synacthen, and then one or two more blood tests 30 to 60 minutes later.¹ Healthy adrenal glands respond with a clear rise in cortisol; damaged or suppressed glands do not. The Endocrine Society calls the short 250 µg test the gold-standard test for diagnosing primary adrenal insufficiency, including Addison's disease,² and in the UK it is how the diagnosis is confirmed in hospital.¹ It is also called the short ACTH stimulation test or, in the US, the cosyntropin test.³

Why the test is done

Adrenal insufficiency means the adrenal glands do not make enough cortisol. In Addison's disease, or primary adrenal insufficiency, the outer layer of the adrenal glands is destroyed; in Western countries about 90 per cent of cases are autoimmune.¹ It is uncommon: about 9,000 people in the UK have the diagnosis, with about 320 new diagnoses a year.¹ Adrenal insufficiency can also be secondary, when the pituitary gland does not make enough ACTH,⁴ or caused by long-term steroid medicines, a risk for the at least 1 per cent of the population on long-term glucocorticoid treatment.⁵

Symptoms of Addison's disease include extreme tiredness, darker patches of skin, loss of appetite and weight loss, tummy pain, salt craving, dizziness on standing, and muscle weakness and cramps.⁶ The usual first step is an 8 to 9am blood cortisol. NICE treats a result below 150 nmol/L as a reason to refer to an endocrinologist, 150 to 300 nmol/L as uncertain, and above 300 nmol/L as making adrenal insufficiency very unlikely.⁷ Specialists then confirm or rule out the diagnosis with a Synacthen test.¹ See the blood cortisol test and low cortisol symptoms.

What happens during the test

1. A blood sample is taken to measure your starting cortisol. A sample for ACTH can be taken at the same time: with confirmed low cortisol, an ACTH level more than twice the upper limit of normal points to a problem in the adrenal glands themselves.²
2. You are given 250 µg of tetracosactide by injection into a vein or a muscle.¹²
3. Blood is taken again after 30 minutes, 60 minutes or both, depending on the local protocol, to see how far cortisol has risen.¹⁴

The test takes about an hour.¹ The injection can sting, and some people feel flushed, nervous or sick for a short while afterwards.⁴ The Endocrine Society notes that the standard test can be done at any time of day,² although your clinic will give you an appointment time and its own instructions.

How to prepare

• Steroid medicines. Hydrocortisone and other steroids can interfere with the cortisol measurement, so you may be asked to stop them for a short time before the test.⁴ Do this only as instructed by the team arranging the test: stopping a steroid suddenly can itself upset cortisol levels.⁸ NICE advises waiting 4 weeks after a steroid injection into a muscle or joint before cortisol testing.⁷
• Oral oestrogen. Contraceptive pills and HRT tablets containing oestrogen raise the protein that carries cortisol, and so raise total cortisol.² In one study, total cortisol was 67 per cent higher in women on oral oestrogen.⁹ Researchers have recommended separate reference limits for women taking the combined pill,¹⁰ and NICE advises stopping oral oestrogen for 6 weeks before cortisol is measured.⁷ Do this only if the doctor arranging the test advises it, and plan contraception with them.
• Food. Some units ask you to fast for about 6 hours beforehand, while others need no special preparation. Follow the instructions you are given.⁴

Understanding the cut-offs

The result is the peak cortisol after the injection. Traditionally, a peak below 500 nmol/L (18 µg/dL) at 30 or 60 minutes indicates adrenal insufficiency,² and NICE's guidance for GPs describes a rise above 500 to 550 nmol/L as normal adrenal reserve.¹ Both stress that the exact cut-off depends on the laboratory's assay.²¹

That caveat matters, because newer, more specific assays give lower numbers than the methods the 500 nmol/L figure came from.

• MedlinePlus gives a normal response as above 15 to 16 µg/dL (430 to 450 nmol/L), depending on the assay and the ACTH dose.⁴
• A 2021 study recommended a 30-minute cut-off of 14 to 15 µg/dL with newer specific immunoassays and mass spectrometry, instead of the historical 18 µg/dL.¹¹ That is about 386 to 414 nmol/L, using 1 µg/dL = 27.59 nmol/L.¹²
• Using mass spectrometry in 138 healthy people, a 2018 study set cut-offs of 412 nmol/L at 30 minutes and 485 nmol/L at 60 minutes.³ With these, 22 per cent (13 of 60) of people who had been classed as having adrenal insufficiency by conventional criteria had a normal test.³
• One UK laboratory treats a 30-minute result above 450 nmol/L as showing adequate adrenal reserve.¹³

A study of 165 healthy volunteers shows how much the method alone matters.¹⁰ The lowest normal cortisol 30 minutes after the injection varied from 420 to 574 nmol/L depending on the analyser, and women on the combined pill needed separate limits.¹⁰

In general, automated immunoassays read 1.08 to 1.36 times higher than mass spectrometry.¹⁴ The practical point is simple: only the cut-off on your own laboratory's report applies, and a result that would fail on one analyser can pass on another.

What your result means

If cortisol rises above your laboratory's cut-off, your adrenal glands responded normally and adrenal insufficiency is unlikely. If it does not, the result points to adrenal insufficiency, which can come from the adrenal glands or the pituitary, and the ACTH level helps tell the two apart.⁴² The test is good but not perfect: in a meta-analysis its sensitivity was 0.92, meaning it picked up about 92 per cent of people with the condition.² Your doctor will interpret it alongside your symptoms and other results.

If you are diagnosed, treatment replaces the missing hormones, and both NICE and the Endocrine Society advise monitoring it through symptoms and examination rather than repeated hormone tests.⁷² You will also be taught sick-day rules, such as doubling your usual daily steroid dose during an illness with fever that needs bed rest or antibiotics, and using injected steroid during prolonged vomiting or diarrhoea.¹⁵ See testing for Addison's disease.

Adrenal crisis: know the warning signs

Adrenal insufficiency can tip into an adrenal crisis, which is a medical emergency. The NHS advises calling 999 if someone with Addison's disease suddenly gets worse or has a fast heart rate, severe dizziness (especially on standing), severe tummy or side pain, muscle weakness, pain or spasms, a headache that will not go away, nausea or vomiting, drowsiness, irritability or confusion, or a seizure or loss of consciousness. Call 999 even after an emergency steroid injection has been given.⁶

NICE advises doctors to suspect an adrenal crisis in anyone critically unwell with low blood pressure, low sodium, low blood sugar or collapse, and to consider it in people at risk with milder symptoms such as lethargy, confusion or weakness.⁷ In a suspected crisis, treatment comes first. The Endocrine Society recommends giving intravenous hydrocortisone straight away, without waiting for test results, while stressing that blood for diagnosis should be drawn before any steroid is given; a Synacthen test can follow once the person is stable.² Emergency guidance is an immediate 100 mg dose of hydrocortisone, followed by 200 mg over 24 hours, with rapid fluids.¹⁵

Why there is no home alternative

A Synacthen test measures how your adrenal glands respond to a stimulus, which no home test can do. A single blood, saliva or urine cortisol shows one moment, and a hair test shows your average cortisol over months; none of them can assess adrenal reserve. The Synacthen test remains the standard for diagnosing adrenal insufficiency, and current guidance does not use hair or other home tests to diagnose it or to monitor treatment.²⁷ If you have symptoms that could mean low cortisol, see a doctor, and read our guide to getting a cortisol test from your doctor.