Radioimmunoassay for luteinizing hormone in human plasma or serum: physiological studies.

It is not practical to quantitate gonadotropin in the blood of normal men and women by utilizing bioassays. We have developed a method for sensitive, precise, and specific radioimmunoassay of luteinizing hormone (LH) in human serum or plasma. Antisera were developed against human chorionic gonadotropin, and one of these was selected for extensive cross-reaction with human LH. Highly purified LH was radioiodinated by the method of Greenwood, Hunter, and Glover. Separation of antibody-bound from free LH-(131)I was accomplished by a double antibody technique. Dose-response curves for the purifed LH, an impure urinary LH preparation, pituitary powder, and LH in plasma were all identical. Immunoassay and bioassay of impure urinary and pituitary gonadotropin preparations in terms of a common standard resulted in an index of discrimination of close to unity. LH levels in plasma from 32 adult men and 30 women outside the midcycle ranged from 0.6 to 3.2 mmug per ml (1 mmug of our laboratory LH standard is equivalent to 8 mU of the Second International Reference Preparation of Human Menopausal Gonadotropin). Levels were remarkably constant in men from day to day and in women except at midcycle, when a sharp peak occurred lasting less than 24 hours. In all women studied who had a midcycle LH peak, mean plasma LH levels during the follicular phase of the menstrual cycle were higher than mean values obtained during the luteal phase. Prepubertal children had detectable plasma LH, and mean values were only slightly less than in adults. Plasma from castrate men or women or postmenopausal women contained 4.5 to 10.5 mmug per ml. Clomiphene treatment of four men resulted in a doubling of plasma LH in 5 days.