Effects of progestin-opposed transdermal estrogen administration on growth hormone and insulin-like growth factor-I in postmenopausal women of different ages

Prior studies in women have shown a positive correlation of endogenous estrogen levels with spontaneous and stimulated GH secretion and basal insulin-like growth factor-I (IGF-I) levels. In postmenopausal women, estrogen replacement therapy (ERT) by the oral route increases basal and GHRH-stimulated GH secretion but decreases basal IGF-I levels. To assess the corresponding effects of transdermal ERT (tERT) on this axis, we administered four 8-week regimens of transdermal 17β-estradiol (Estraderm; 0, 50, 100, or 150 μg/day) combined with oral medroxyprogesterone acetate (10 mg each day) during weeks 3-4 and 7-8 of each 8-week regimen (except placebo) to 28 healthy nonobese postmenopausal women, aged 45.3-71.8 yr. Basal levels of estradiol (E₂), GH, and IGF-I as well as GH responsivity to bolus iv administration of GH-releasing hormone-(1-44) (1 μg/kg), were measured before tERT and at weeks 6 and 8 of each regimen; estrone (E₁) levels were measured before tERT and at week 6 of each regimen. Before tERT, age was inversely correlated with both the peak GH response to GHRH (r = -0.43; P < 0.02) and basal IGF-I levels (r = -0.37; P < 0.05), but not with basal E₂, E₁, or GH levels. There were progressive increases in plasma E₂ and E₁ levels with increasing doses of tERT (P = 0.0001), independent of age (P > 0.2) and body mass index (P > 0.2). Mean basal GH and IGF-I levels were not altered significantly by tERT or medroxyprogesterone acetate. Peak and integrated GH secretory responses to exogenous GHRH decreased with increasing tERT dose (P < 0.01) in both younger and older postmenopausal women. Our findings suggest that the known effects of tERT on bone and other tissues are not mediated via increases in circulating levels of immunoreactive GH or IGF-I, but do not preclude the possibility of tERT-induced increases in the biological activity or paracrine action of IGF-I.