PHYSIOLOGICAL ESTRADIOL ADMINISTRATION DOES NOT CHANGE THE BIOLOGICAL TO IMMUNOLOGICAL RATIO OF LH IN SERUM FROM POSTMENOPAUSAL WOMEN

OBJECTIVE We investigated the effect of chronic physiological concentrations of oestradiol on the bioactivity (B) and immunoactivity (I) of LH in serum using radioimmunoassay (RIA) and a sensitive immunoradiometric (IRMA) assay. DESIGN Twelve post-menopausal, hysterectomized women received a 50-mg subcutaneous E2 implant, in the absence of progestogen treatment. Pretreatment serum samples were obtained on two occasions for measurement of E2 and FSH by radioimmunoassay and for measurement of LH by bioassay (BIO), immunoradiometric assay and radioimmunoassay. Further serum samples were obtained monthly for 6 months for assay as above. RESULTS The serum E2 level (basal levels <70 pmol/l) rose following implant (P < 0.01) peaking at 1 month (mean 454, range 356-580 pmol/l) and remaining within premenopausal physiological levels for 6 months. There were falls in FSH, LH-RIA, LH-IRMA and LH-BIO (P < 0.001). There was no change in the B:I ratio for B:I-RIA or B:I-IRMA for LH which were 18 and 3.3, respectively pretreatment and 1.7 and 2.9 after 1 month's treatment. CONCLUSION The treatment of post-menopausal women with 50-mg E2 subcutaneous implants produces physiological premenopausal E2 levels for at least 6 months. There is no change in the B:I ratio of LH using either the RIA or IRMA during chronic E2 treatment.