[Failure of synthcen to diagnose corticotropin insufficiency]

It is commonly believed that a normal cortisol response to exogenous ACTH (synacthen test) reliably indicates, with the exception of states of acute ACTH deprivation, the integrity of hypothalamic-pituitary-adrenocortical function. Published evidence has shown that the synacthen test is not sensitive enough to reveal partial ACTH deficiency leading to a subnormal ACTH response to stress. We report a patient who maintained a normal response to exogenous ACTH stimulation despite symptomatic chronic ACTH deficiency in unstressed conditions. A normal cortisol response in the synacthen test does not therefore exclude the possibility of clinically relevant ACTH deficiency.