Antagonism of oestrogen-induced prolactin release by medroxyprogesterone acetate.

Previous studies conducted at our clinic suggested that the administration of hormone replacement therapy (HRT) in postmenopausal women could result in the inhibition of oestrogen-induced prolactin (PRL) release. The aim of this study was to determine how the pituitary function is affected by the sequential addition of medroxyprogesterone acetate (MPA) to oestrogen replacement therapy. Twenty-one postmenopausal women receiving no other medication were treated with a standard dose (0.625 mg/day) of conjugated equine oestrogens (CEE) for a period of 24 days, plus 5 mg/day MPA added sequentially during the last 12 days of the oestrogen therapy. Blood samples were collected before treatment, during oestrogen and oestrogen-progestogen administration and after cessation of treatment. Follicle-stimulating hormone (FSH), luteinizing hormone (LH), 17 beta-oestradiol (E2) and PRL levels were studied. During treatment gonadotrophin concentrations decreased significantly, while after cessation of HRT the levels of FSH and LH increased. These gonadotrophin fluctuations indicated a sharp rise in E2 levels during therapy and a significant decrease during the treatment-free period. PRL levels were found to be higher during CEE therapy, but they fell when patients received CEE in combination with MPA. These observations suggest that the role of progestogens in a variety of experimental and clinically relevant situations needs to be investigated not only as regards their direct action but also their modulation of the effect of oestrogen.