Reproducibility of the cortisol response to stimulation with a low dose of ACTH(1-24): the effect of basal cortisol levels and comparison of low-dose with high-dose secretory dynamics.

We compared the reproducibility and repeatability of the acute adrenal response to low doses (90 and 500 ng/1.73 m2) of Synacthen (ACTH(1-24)) with that of the standard dose (250 micrograms/1.73 m2). We also examined the effect of basal cortisol levels on peak values achieved after stimulation with a low dose. ACTH(1-24) was given to six male volunteers: 90 ng/1.73 m2 twice at 90-min intervals on day 1, and 90 and 500 ng/1.73 m2 once on day 2 and 250 micrograms/1.73 m2 once on day 3. The rise in serum cortisol concentration with repeated low doses of ACTH was not attenuated (161 +/- 49 (S.D.) nmol/l on initial vs 150 +/- 41 nmol/l on repeat stimulation; P = 0.5) and this was reproducible (161 +/- 49 nmol/l on day 1 vs 148 +/- 15 nmol/l on day 2; P = 0.6). A dose of 500 ng ACTH(1-24)/1.73 m2 produced a maximal adrenal response in that the rise in serum cortisol concentration at 20 min was identical with that produced at the same time by the standard dose of 250 micrograms/1.73 m2. There was a strong positive correlation between the basal cortisol level and peak cortisol concentration after low-dose ACTH stimulation (r = 0.93, P < 0.001) but not between the basal cortisol level and the incremental rise (r = -0.1, P = 0.69). These results suggest that the cortisol response to low-dose ACTH stimulation is reproducible and not attenuated by repeat stimulation at 90-min intervals.(ABSTRACT TRUNCATED AT 250 WORDS)