Circulating levels of growth hormone, insulin-like growth factor-I and growth hormone binding protein in normal women of advanced reproductive age

Objective. Women experience an age-related decline in fertility despite regular ovulatory cycles and normal production of ovarian steroids. Growth hormone and IGF-I are both reported to decline with age, and there is evidence that both hormones promote intraovarian actions of gonadotrophins. The purpose of this study was to characterize circulating levels of GH and IGF-I in normal, older reproductive age women with ovulatory cycles. Design. Prospective, controlled. Patients. Twenty-eight regularly cycling older (n=16) and younger (n=12) women were recruited for daily blood sampling throughout a menstrual cycle. Measurements. Serum obtained from daily blood sampling was analysed for LH, FSH, oestradiol (E₂) and progesterone (P). Serum obtained from frequent sampling during the admission was analysed for pulsatile GH secretion. IGF-I and GH binding protein (GHBP) were also measured in subsets of the two age groups. Results. AIl subjects exhibited normal patterns of LH, FSH, E₂ and P consistent with ovulatory cycles. There were no differences between the two age groups in integrated 24-hour GH secretion or in GH pulse amplitude or frequency. There were no differences in GH secretion between the early follicular and midluteal phases when data were combined for the two subject groups. Plasma concentrations of IGF-I were significantly lower throughout the cycle in the older women. There were no significant differences in levels of GHBP across the cycle or between the two age groups. Conclusions. IGF-I decreases with age in women without identifiable changes in the amount or pattern of GH secretion or in circulating GHBP concentrations. Decreased IGF-I production may be related to decreased ovarian gonadotrophin sensitivity in older reproductive age women.