Expansion
Measure cortisol before and after synthetic ACTH
Expansion
Method: measure baseline cortisol, give 250 micrograms of synthetic ACTH (tetracosactide) intramuscularly or intravenously, then measure cortisol at 30 and 60 minutes.
Interpretation: a peak cortisol above the laboratory threshold (commonly 420 to 550 nmol/l depending on the assay) excludes adrenal insufficiency.
Points of interpretation
- A failed response confirms adrenal insufficiency but does not distinguish primary from secondary; a paired ACTH level does that, being high in primary and low in secondary
- In recent pituitary damage, such as within 4 to 6 weeks of surgery or apoplexy, the adrenals have not yet atrophied and may still respond, so the test can be falsely reassuring. An insulin tolerance test or a repeat later is needed
- Hydrocortisone and prednisolone cross-react in the assay and must be withheld; dexamethasone does not, so it can be given to a sick patient while awaiting the test
The practical rule in a shocked patient with suspected adrenal crisis is to take a random cortisol and ACTH, treat immediately with dexamethasone or hydrocortisone, and formally test later.