Diurnal Variation of Growth Hormone Responses to Acutely Administered Dexamethasone in Healthy Male Volunteers

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Abstract
Acute administration of exogenous corticosteroids stimulates growth hormone (GH) release. Physiological fluctuations of plasma cortisol appear to have an effect on the circadian release of GH. The purpose of this study was to examine the effects of the diurnal variation of plasma cortisol on dexamethasone (DEX)-induced GH responses in healthy volunteers. Six healthy male subjects were studied using a randomised cross-over design. Subjects were tested twice, with an interval of two weeks between tests, at 9.00 h and 14.00 h. After baseline levels for GH and cortisol were taken, subjects received 4 mg of oral DEX and blood samples were drawn at + 60, + 180, + 240 and + 300 minutes later. GH levels peaked 3 hours post-DEX administration and were greater in the morning than in the afternoon. Mean +/- SEM delta GH values at 9.00 h and 14.00 h were 14.1 +/- 2.7 ng/ml and 6.9 +/- 1.6 ng/ml, respectively (t = 2.36, df = 1.10, p < 0.04). DEX-induced GH release appears to undergo a diurnal variation.

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