Synacthen Test (Investigation Guide)
A Synacthen test (ACTH stimulation test) checks how well your adrenal glands respond to a stimulating hormone, and is used to diagnose or exclude Addison's disease.
Why it's done
- a single cortisol blood test is not enough, since levels fluctuate through the day
- a Synacthen test gives a clearer picture of adrenal function
- confirms or excludes adrenal insufficiency
What we offer
- a baseline cortisol sample followed by a Synacthen injection and further samples at 30 and 60 minutes
- clear guidance on any medication to pause beforehand
- results explained the same day where possible
Preparation and procedure
The testing unit will advise about steroid medicines because hydrocortisone and some other treatments can affect cortisol measurement. Baseline blood is taken, synthetic ACTH is given, and cortisol is measured again at specified times. The test is generally well tolerated, though brief flushing, nausea or light-headedness may occur.
Understanding the result
Interpretation depends on the laboratory assay, timing, clinical context and recent steroid exposure. A normal short test does not answer every question about pituitary ACTH reserve, so further assessment may occasionally be required.
UK-wide video review
Results can usually be reviewed securely by video, while the test itself requires an appropriate local facility. Suspected adrenal crisis requires emergency treatment and must not be delayed for testing.