Measurement of human luteinizing hormone in plasma by radioimmunoassay

Abstract

The recent isolation of highly purified human pituitary luteinizing hormone (LH) has permitted the development of a sensitive and specific radioimmunoassay for this hormone in plasma. Results of this immunoassay system employing anti-LH serum agree closely with previous reports for the measurement of plasma LH in which immunoassays employing cross-reactive antisera to human chorionic gonadotropin were used. The immunoassay and bioassay of LH in several crude and partially purified pituitary and urinary extracts show acceptable agreement. The sensitivity of the LH immunoassay (0.2 mμg/ml) is adequate to measure LH levels in almost half of all prepuberal children and in all but a few normal adults. A small, but significant, rise in plasma LH level occurs at pubescence in both boys and girls. In women, plasma LH level varies with both age and the phase of the menstrual cycle. The mean LH concentration in nine normal women during the follicular phase (1.2 mμg/ml was found to be significantly higher than during the luteal phase (1.0 mμg/ml). At midcycle, the mean peak LH level was 10.2 mμg/ml. In a large group of normal women, the mean plasma LH concentration rose significantly at menopause to a level of 5.8 mμg/ml during the fifth decade and 10.5 mμg/ml during the seventh decade. A small, but significant, rise in plasma LH concentration also occurred in men from the third and fourth decades (0.7 mμg/ml to the seventh and eighth decades (1.7 mμg/ml). Both estrogen and testosterone suppress plasma LH levels, but marked variation in response exists. The immunoassay serves as a useful diagnostic tool in evaluating men with gonadal failure, amenorrheic women of reproductive age, and postmenopausal women suspected of hypopituitarism. From the half-time disappearance of LH-¹³¹I in plasma (mean 69 min) and the calculated volume of distribution (2.5-2.8 liters) it has been determined that approximately 30 μg of LH is secreted per day in men, and in women except at midcycle, at which time the release of LH is estimated to be 10-15 times this basal rate.