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Development of a radioligand receptor assay for measuring follitropin in serum: application to premature ovarian failure.

We have developed a radioligand receptor assay (RRA) with sufficient sensitivity and specificity for quantifying follitropin (FSH) in unextracted serum samples. Standard curves prepared by adding pituitary FSH to either buffer or gonadotropin-free serum were parallel and statistically indistinguishable in this assay, whereas gonadotropin-free serum alone had no activity. Cross-reactivity with related pituitary hormones was negligible. Pituitary FSH was calibrated with commonly used reference preparations so that RRA results could be compared with RIA results for identical standards. The patterns in daily blood samples in six normal menstrual cycles were similar by both methods. The mean RIA:RIA ratio in both the follicular and luteal phases was between 0.6 and 0.7, and at mid-cycle decreased to 0.48, suggesting an alteration of isohormone composition at mid-cycle. In 27 women with premature ovarian failure, RRA:RIA ratios ranged from below the RRA minimum detectable dose to 4.6, suggesting that immunoreactive FSH might not be capable of binding to the FSH receptor in some patients, whereas in patients with high RRA:RIA ratios, circulating inhibitors of FSH receptor binding might be present and perhaps contributing to the observed ovarian failure. Use of this RRA in conjunction with RIA and in vitro bioassays may better define the relative contribution of FSH isohormones, autocrine or paracrine modulators of FSH bioactivity, and FSH-receptor binding competitors to the "total FSH biological signal" as detected by the gonadal FSH receptor.