Alterations in the Pulsatile Properties of Circulating Growth Hormone Concentrations during Puberty in Boys*

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Abstract
To investigate the mechanisms subserving physiological alterations in circulating GH concentrations during puberty, we assessed the GH pulse characteristics of 60 24-h serum GH profiles obtained from healthy male volunteers of normal stature (aged 7-27 yr) whose physical development spanned the entire pubertal range. Subjects were divided into five study groups based on degree of sexual maturation. The mean 24-h concentration of GH was greater in late pubertal boys than in all other groups (P < 0.001). This elevation primarily reflected a greater size, rather than number, of GH pulses, whether assessed as mean GH pulse area (P = 0.004 vs. all other groups), mean GH pulse amplitude (P = 0.001), or sum of the GH pulse areas (P < 0.001). GH pulse frequency was indistinguishable among all groups (P > 0.05). However, circadian GH rhythms varied significantly in amplitude and mean values (but not in phase) throughout puberty. Plasma insulinlike growth factor-I levels were greatest in the late pubertal boys (1.98+−0.15 U/mL) and remained elevated in the postpubertal group (1.44+−0.18). The mean value for the adult men (0.74+−0.06) was indistinguishable from that of prepubertal boys (0.90+−0.13). In addition, all assessed characteristics of GH pulses and circadiam rhythms in adults were equal to or less than corresponding values in prepubertal boys. We conclude that circulating GH concentrations transiently increase during mid-to late puberty in normal boys, primarily through augmentation of the size of GH pulses, but return to or below prepubertal levels during early adulthood.