A short Synacthen test, also called an ACTH stimulation test, checks whether your adrenal glands can produce cortisol when they are asked to. It is the standard test for suspected adrenal insufficiency, which includes Addison's disease and the adrenal suppression that can follow long courses of steroid medication. It is usually arranged after a low or borderline morning cortisol result.
Why the test is done
Cortisol is released in response to ACTH, a hormone from the pituitary gland. A single cortisol measurement can be misleading because levels change through the day and fall with stress, illness and some medicines. Synacthen is a synthetic form of ACTH. Giving a small dose and measuring cortisol before and after shows directly whether the adrenal glands can respond, which is the question that matters.
Typical reasons for the test include persistent tiredness with low blood pressure or dizziness on standing, unexplained weight loss, salt craving, skin darkening, a low morning cortisol, or a planned reduction in long-term steroid tablets or inhalers.
What happens on the day
- The test is done in the morning, usually starting between 8am and 9.30am, because that is when reference values apply.
- A baseline blood sample is taken for cortisol, and sometimes ACTH.
- A 250 microgram dose of Synacthen is injected into a muscle or vein.
- A second cortisol sample is taken 30 minutes later, and some protocols add a 60 minute sample.
- You can eat and drink normally beforehand unless you have been told otherwise, and the whole visit takes about an hour.
Side effects are uncommon and usually mild: brief flushing, nausea or a warm feeling around the injection site. Tell the clinic before the test if you have ever reacted to Synacthen, or if you are pregnant or breastfeeding.
Medicines that affect the result
Steroid medicines are the main issue. Hydrocortisone and prednisolone cross-react with the cortisol assay and also suppress the adrenal response, so the clinician who arranges the test will tell you whether to hold a dose and for how long. Never stop steroids without advice. Oral oestrogen, including the combined pill and HRT tablets, raises the protein that carries cortisol and can push results up, so mention it when you book.
What the results mean
Laboratories report the baseline and stimulated cortisol values and apply a cut-off, commonly a stimulated cortisol above about 420 to 450 nmol/L depending on the assay. A stimulated value above the cut-off means the adrenal glands responded normally and adrenal insufficiency is unlikely. A value below it means the response was inadequate and needs specialist review to work out whether the problem is in the adrenal glands themselves or in the pituitary signal to them. A borderline result is usually repeated or followed by further tests rather than acted on alone.
The cut-off depends on the assay the laboratory uses, which is why the number on your report matters less than the interpretation printed beside it. Blood London reports state the cut-off used.
Having the test privately
Blood London offers the short Synacthen test at 117a Harley Street on a self-request basis. Because the test needs a timed injection and two draws, it is arranged by enquiry rather than instant online booking so a morning slot can be reserved. A morning cortisol test can be booked online first if you have not had one. Bring a list of your current medicines, including any steroid inhalers, creams or tablets.
Common questions
Do I need to fast?
No. Eat and drink normally unless the clinician who arranged the test says otherwise. Fasting does not improve the result.
Can I drive afterwards?
Yes. The test does not cause drowsiness. A few people feel briefly light-headed after the injection, so wait until you feel normal before leaving.
How long do results take?
Cortisol results are usually available within one to two working days. The report includes the baseline and stimulated values and the laboratory cut-off.