The Impact of Dose and Route of Estrogen Administration on the Somatotropic Axis in Normal Women

Abstract

Oral estrogen therapy has reliably been found to reduce levels of serum IGF-I and increase mean 24-h GH levels in postmenopausal women as well as increase GH requirements in patients with GH deficiency. It is thought to act by inhibiting GH-stimulated IGF-I secretion, thus resulting in diminished feedback at the hypothalamic-pituitary axis and, hence, increased GH levels. In contrast, the administration of transdermal estrogen has variably been found to reduce, not change or increase, levels of serum IGF-I. We sought to clarify the effect of transdermal estrogen on the GH/IGF-I axis by using the IGF-I generation test, in which the acute response to a bolus dose of GH is examined. Nine healthy postmenopausal women received three different formulations of estrogen: oral estradiol (1 mg every 12 h), transdermal estradiol (50 μg/d), and transdermal estradiol (200 μg/d) for a 6-wk period in random order, separated by an 8-wk washout period. At the start of the study and in the last week of each estrogen formulation treatment, subjects underwent an IGF-I generation test. Oral estradiol reduced baseline (P < 0.05) and GH stimulated (P < 0.05) IGF-I levels, and GH stimulated IGF-binding protein-3 (IGFBP-3) levels (P < 0.05). High dose transdermal estrogen did not affect basal levels of IGF-I or IGFBP-3, but reduced the response of these GH-dependent peptides to GH stimulation (P < 0.05). Low dose transdermal estrogen did not alter either baseline or peak IGF-I levels, but reduced the peak IGFBP-3 response to GH stimulation (P < 0.05). Estradiol levels were lower during both transdermal estrogen preparations than during oral estrogen. It has been suggested that the effect of estrogen on responsiveness to GH is limited to that administered by the oral route. We have demonstrated that transdermal estrogen also has a significant impact on responsiveness to GH despite achieving levels of circulating estrogen lower than those achieved by oral estrogen replacement.