Luteinizing hormone--releasing hormone levels in human plasma: a radioimmunoassay method.

A sensitive double-antibody radioimmunoassay, developed to measure luteinizing hormone--releasing hormone (LHRH) levels in fetal brain tissues, has been modified to quantitate LHRH in unextracted plasma samples. Pure, synthetic LHRH was iodinated with 125I for tracer and was also used as standard. The antibody utilized was specific for LHRH and did not cross react with other hypothalamic or pituitary hormones. The separation of free from antibody-bound tracer was performed by dextran-coated charcoal. The sensitivity of the assay was 2.5 pg/tube. The intra-assay and interassay coefficients of variation were 5.9 and 6.1% for 50 pg/ml and 11.8 and 14.9% for 25 pg/ml samples. Recovery of known amounts of LHRH added to plasma samples that had no detectable hormone was 96.4 +/- 8.8%. Whether these samples were previously prepared and kept frozen at -20 degrees C or freshly prepared, the recovery of LHRH was consistent and quantitatively stable in each assay. Serial determinations of LHRH in plasma obtained from ovulatory volunteers averaged between 20.5 +/- 2.1 pg/ml in the follicular phase and 19.4 +/- 4.5 pg/ml in the luteal phase. At midcycle, coincident with the luteinizing hormone surge, LHRH levels averaged 17.6 +/- 4.4 pg/ml.