Age-Related Change in the Twenty-Four Hour Spontaneous Secretion of Growth Hormone

Spontaneous growth hormone secretion rates and secretory patterns were studied in a group of normal prepubertal children, adolescents and young and older adults by determining the concentration of growth hormone in plasma samples obtained at 20-min intervals over a 24-hr period. Prepubertal children secreted growth hormone only during sleep and not while awake and had a mean secretion rate of 91 μg/24 hr (range 54–122 μg/24 hr). Mean duration of secretion was 110 min/24 hr (range 60–180 min/24 hr). Adolescents secreted growth hormone during both awake and sleep periods and had a mean secretion rate of 690 μg/24 hr (range 391–1406 μg/24 hr). Mean duration of secretion was 226 min/24 hr (range 100–300 min/24 hr). Secretion rates in young adults (21–41 yr) averaged 385 μg/24 hr (range 215–769 μg/24 hr). Average duration of secretion was 133 min/24 hr (range 100–300 min/24 hr). Growth hormone was secreted during both awake and asleep periods but the number of secretory episodes was less than in adolescents. In older adults (42–62 yr) the total 24-hr secretion of growth hormone decreased and approached zero in three of five studies. These data clearly demonstrate an age related change in the spontaneous secretory rate and secretory pattern of growth hormone.