Growth hormone secretory profiles: variation on consecutive nights

To evaluate the reproducibility of overnight growth hormone (GH) testing and the effect of daytime administration of levodopa and clonidine on overnight GH secretion, we examined consecutive 12-hour overnight GH profiles of 48 short subjects, ages 5 to 16 years, who had GH stimulation testing with levodopa and clonidine. In six subjects (12%) the overnight pool GH concentration on the second night increased by greater than 100% from the first-night result (night-to-night changes of +1.2 to +5.2 ng/ml). In the remaining subjects, night-to-night changes in pool GH concentrations ranged from -1.2 to +1.8 ng/ml (-60% to +88% changes from the first night value). Night-to-night changes were less than 25% of the first-night value in 17 subjects (35%), 25% to 50% in 18 subjects (38%), and 50% to 100% in 7 subjects (15%). Night-to-night changes in pool GH concentrations correlated with differences in peak nighttime GH concentrations but not with differences in duration of observed sleep. There was no discernible effect of daytime levodopa-clonidine administration on overnight pool GH concentrations. These results demonstrate the potential for night-to-night variation in overnight GH profiles and suggest the need for some means of confirming that overnight GH testing reflects normal physiologic GH secretion. Without such confirmation, the results from a single overnight GH profile should be interpreted with caution.