Boron causes elevation in estrogen (17 beta-estradiol) and testosterone, reduction in urinary excretion of magnesium and calcium, increases plasma copper, SOD, and ceruloplasmin, (the last three are also raised by estrogen therapy, but by a lesser degree is boron is insufficient), and "it is possible that high dietary intakes of boron or molybdenum could regulate the rate of catabolism, or even the metabolic fate of the major estrogens." (i.e., could prevent methylation of estrogen by COMT)

sources:
http://www.nlm.nih.gov/medlineplus/druginfo/natural/894.html
http://www.ncbi.nlm.nih.gov/pubmed/3678698
http://www.ithyroid.com/boron.htm