The variations of plasma corticosterone/cortisol ratios following ACTH stimulation or dexamethasone administration in...

A radioimmunoassay for human plasma corticosterone has been developed. Plasma corticosterone increased 4.83 times as much as basal value at 60 min after an im injection of 0.25 mg synthetic beta1-24 ACTH (Cortrosyn) in normal subjects, whereas plasma cortisol increased 2.12 times as much at 60 min. And basal corticosterone/cortisol ratio of 0.053 +/- 0.017 increased to 0.116 +/- 0.022 (P less than 0.001) after ACTH. This might be mainly due to a larger increment of corticosterone than that of cortisol after ACTH. Corticosterone decreased to 36.7% of basal value at 4 h after 1 mg dexamethasone administration in normal subjects, whereas cortisol decreased to 13.9% of basal value at 4 h. The basal corticosterone/cortisol ratio of 0.059 +/- 0.020 increased to 0.137 +/- 0.055 (0.001 less than P less than 0.01) after dexamethasone administration. This may have been mainly due to a more effective suppression of cortisol than that of corticosterone after dexamethasone.