Diminished function of the somatotropic axis in older reproductive-aged women

Circulating GH and insulin-like growth factor-I (IGF-I) levels in adults generally fall with age. Studies in aging women have rarely controlled for menstrual cycle stage or status or body mass index. We hypothesized that GH and IGF-I levels in reproductive-aged women fall with age despite the stimulatory effects of endogenous estradiol (E2). Eight older reproductive-aged women (aged 42-46 yr) with regular menses, of normal weight, and in good health were compared to a group of eight young control subjects (aged 19-34 yr). Daytime frequent blood sampling was performed in the early follicular phase of the menstrual cycle to characterize pulsatile GH and LH concentrations. Pooled samples were also analyzed for IGF-I, E2, progesterone, and FSH levels. Older reproductive-aged women had lower 12-h integrated daytime GH concentrations (mean +/- SE, 171 +/- 35 vs. 427 +/- 130 micrograms min/L; P = 0.036) than younger controls and a strong trend for lower IGF-I levels (22.7 +/- 2.1 vs. 31.3 +/- 3.5 nmol/L; P = 0.055) than younger controls despite having higher circulating E2 on the day of sampling (368 +/- 51 vs. 167 +/- 20 pmol/L; P = 0.002). We conclude that older reproductive-aged women have lower daytime GH concentrations than younger controls despite having higher E2 levels on the day of sampling and overall normal gonadal hormone parameters.