Estrogenic action of tamoxifen in women treated with leutinizing hormone releasing-hormone agonist goserelin [zoladex]: lack of shrinkage of uterine fibroids

Six premenopausal women with uterine fibroids were treated with a combination of tamoxifen 20mg/d and Zoladex 3.6mg every 28 days for a total of 24 weeks. Results were compared with those from six women matched for pretreatmented uterine volume who had been treated with zoladex alone. During combined therapy, plasma and urinary estrogen concentrations were significantly lower than during Zoladex alone, whereas sex hormone binding globulin concentrations were significantly higher. Plasma LH and FSH concentration were both suppressed in contrast with results during Zoladex alone when FSH levels remained with the pretreatment range. None of the women on combined therapy bled in response to the endocrine change of the internal treatment cycle. Despite this profound pituitary-ovarian suppression. There was no significant change in uterine volume during combined therapy. The results suggest that tamoxifen is acting as an estrogen agonist in women rendered hypoestrogenic with luteinizing releasing hormone agonist. We failed to demonstrate any decrease in size of uterine fibroids when the antiestrogen tamoxifen was administered continuosly as a single agent. However, this lack of response was attributable to significant elevation of estrogen concentration during tamoxifen therapy