Corticotrophin stimulation test: inverse correlation between basal serum cortisol and its response to corticotrophin

Serum cortisol response to synthetic corticotrophin (B1-24-ACTH) was evaluated in 21 normal subjects. There was significant negative correlation (r = -0.633, P less than 0.001) between the basal serum cortisol and the increase in serum cortisol level in response to ACTH. Two subjects had an increase in serum cortisol of less than 194 nM/1 (7.0 microgram/100 ml) and two additional subjects had an increase of less than 278 nM/l (10.0 microgram/100 ml). All subjects had an absolute peak value greater than 583 nM/l (21.0 microgram/100 ml) at 30 or 60 min post-ACTH. Inverse correlation between basal serum cortisol and the increase in serum cortisol suggests that in patients who have high basal serum cortisol levels, endogenous ACTH may already have reduced the cortisol reserve of the adrenal gland.