Your morning cortisol came back low or borderline, and now a doctor has mentioned a short Synacthen test. It sounds more involved than it is: one morning visit, one injection and two blood samples.
The 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.
Blood London offers the short Synacthen test at 117a Harley Street on a self-request basis. It costs £346.25 plus the £50 blood draw. Because it needs a timed injection and two draws, it's arranged by enquiry so a morning appointment can be reserved for you. If you haven't had one yet, a morning cortisol test can be booked online first.
Why the test is done
Cortisol is released in response to ACTH, a hormone from the pituitary gland. A single cortisol measurement can mislead, because levels change through the day and fall with stress, illness and some medicines. Synacthen is a synthetic form of ACTH. A small dose, with cortisol measured before and after, shows directly whether the adrenal glands can respond, which is the question that matters.
Typical reasons include persistent tiredness with low blood pressure or dizziness on standing, unexplained weight loss, salt craving or skin darkening. The test is also used after a low morning cortisol, or before 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.
Bring a list of your current medicines, including any steroid inhalers, creams or tablets. 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. The clinician who arranges the test will tell you whether to hold a dose and for how long, so keep taking your steroids until they've advised you. 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. It 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 before anything is decided.
Because the cut-off depends on the laboratory's assay, the interpretation printed beside your number matters more than the number itself. Blood London reports state the cut-off used.
Frequently asked questions
Do I need to fast?
No. Eat and drink normally unless the clinician who arranged the test says otherwise. Fasting makes no difference to the result.
Can I drive afterwards?
Yes. The test doesn't 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.