Tamoxifen Prolongs Luteal Phase in Premenopausal Women but Has No Effect on the Size of Uterine Fibroids

Tamoxifen (20 mg per day) was administered for at least 3 months to six premenopausal women with uterine fibromyomata. In all the women there was an increased variability in the length of the menstrual and ovarian cycle associated with a significant lengthening of the luteal phase (16.9 +/- 3.5 vs 12.5 +/- 1.5 days, mean +/- SD, P less than 0.02). The concentration of oestradiol and progesterone in plasma and their metabolites in urine (oestrone glucuronide and pregnanediol) increased reflecting multiple follicular development and ovulation. There was a significant rise in the concentration of FSH during the luteal phase of the cycle (8.0 +/- 2.9 vs 2.0 +/- 1.7 U/l, P less than 0.01). There was no change in the size of the uterine leiomyomata. These results demonstrate that the antioestrogen tamoxifen results in an increased secretion of gonadotrophins by antagonizing the effects of oestradiol at the hypothalamus and anterior pituitary. The lengthening of the luteal phase suggests oestradiol may play a role in the mechanism of luteal regression either locally or via feedback effect on gonadotrophins.