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 endogenous GH by somatostatin. GH was sampled every 10 min and the t1/2 for GH was determined by deconvolution analysis (two compartments). The GH elimination half-life was shorter in the morning: for endogenous GH, t1/2 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 estradiol (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-kilodalton GH was shorter compared to endogenous GH (P < 0.002), and diurnal variation was even more pronounced: t1/2 was 14 +/- 1.0 min in the morning and 19 +/- 1.0 min in the evening (P < 0.01). These effects were not due to differences in GH distribution volumes. The half-life of exogenous 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 rate of GH disappearance from the circulation depends on both diurnal mechanisms as well as the source or structural composition of the hormone. Body size and sex steroids contribute to the variability of GH clearance in healthy man.