Insulin-like growth factor I (IGF-I) serum level modifications during transdermal estradiol treatment in postmenopausal women: a possible bimodal effect depending on basal IGF-I values.

The objective of this study was to determine the influence of transdermal estradiol administration on insulin-like growth factor I (IGF-I) serum level in a series larger than those published to date. Thirty-nine postmenopausal women with vasomotor symptoms were studied; blood samples (after overnight fast) were obtained just before and at the 6th month of treatment with transdermal estradiol 0.05 mg/day, and serum levels of IGF-I, growth hormone and sex hormone binding globulin (SHBG) were evaluated. Sixteen of the 39 women did not show variations of IGF-I values (group A), while 11 showed an increase (group B) and 12 showed a decrease (group C) by at least 20% with transdermal estradiol treatment. IGF-I basal levels were higher in group C as compared to group A (p < 0.05) and to group B (p < 0.01), intermediate in group A, and lower in group B. Group C showed a significant increase of SHBG values with transdermal estradiol treatment. Transdermal estradiol seems to induce a bimodal effect on IGF-I serum levels, depending on IGF-I basal values. This could be caused by a different responsivity to estrogen action on the liver (the major site of circulating IGF-I production) and also, possibly, by a different degree in insulin sensitivity changes caused by estrogen.