The synacthen test is used to test adrenal reserve. Synacthen is tetracosactrin, the first 24 amino acids of ACTH.
Short synacthen test:
- take a basal sample for cortisol
- give 250 microgramme Synacthen i.v. or i.m.
- sample for cortisol are taken at 30 mins and 60 mins
In healthy individuals, the basal plasma cortisol should exceed 170 nmol per litre and rise to at least 580 nmol per litre. The hypoadrenal patient is unable to raise their serum cortisol in response to synacthen.
A depot synacthen test may be performed if the short procedure is equivocal and Addison's disease is suspected:
- basal plasma cortisol is measured
- 1 mg of synacthen is given intramuscularly
- samples for cortisol are taken at 1, 4, 8, and 24 hours
Plasma cortisol fails to exceed 580 nmol per litre in Addisons disease but in secondary hypoadrenalism, a delayed but normal cortisol response may be observed.