DIURNAL-VARIATION IN THE ELIMINATION RATE OF HUMAN GROWTH-HORMONE (GH) - THE HALF-LIFE OF SERUM GH IS PROLONGED IN THE EVENING, AND AFFECTED BY THE SOURCE OF THE HORMONE, AS WELL AS BY BODY-SIZE AND SERUM ESTRADIOL

The half-lives of endogenous and exogenous (biosynthetic monomeric) GH were compared in the morning and evening in healthy young men (n = 10 ). In group A, a bolus of GHRH was injected either at 0800 or at 2000 h, whereas in group B hGH was injected iv after suppression of endogen ous GH by somatostatin. GH was sampled every 10 min and the t1/2 for G H was determined by deconvolution analysis (two compartments). The GH elimination half-life was shorter in the morning: for endogenous GH, t 1/2 was 23 +/- 1.1 min (mean +/- SE) in the morning compared to 26 +/- 1.7 min in the evening (P < 0.02). T1/2 correlated negatively with es tradiol (r = -0.78; P < 0.01) and positively with sex hormone-binding globulin (r = 0.71; P < 0.03). The half-life of exogenous 22-kilodalto n GH was shorter compared to endogenous GH (P < 0.002), and diurnal va riation was even more pronounced: t1/2 was 14 +/- 1.0 min in the morni ng and 19 +/- 1.0 min in the evening (P < 0.01). These effects were no t due to differences in GH distribution volumes. The half-life of exog enous GH was significantly affected by weight (r = -0.8; P < 0.01) and height (r = -0.67; P < 0.05). We conclude that in young males, the ra te of GH disappearance from the circulation depends on both diurnal me chanisms as well as the source or structural composition of the hormon e. Body size and sex steroids contribute to the variability of GH clea rance in healthy man.