Immunoreactive relaxin in human serum during the menstrual cycle.

Immunoreactive relaxin was measured in serum obtained at the same time each day from one woman during a normal menstrual cycle and eight other women using selected bleeding schedules. In addition to relaxin immunoactivity, serum luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, estradiol 17beta, and progesterone were measured in the same samples. The data was combined according to the day of the LH peak. Serum relaxin immunoactivity was found to be high over the period of menstruation, its concentration then decreased until the middle of the follicular phase when a peak equal in amplitude to the menstrual peak occurred. Relaxin immunoactivity remained fairly constant throughout the preovulatory LH surge; however, as the LH levels dropped, relaxin immunoactivity increased reaching maximal levels for the whole cycle. Relaxin immunoactivity then increased gradually over the luteal phase and reached a peak when progesterone levels were declining; it then dropped to low levels until menses ensued. It is not possible with the present radioimmunoassay to determine whether the immunoactive relaxin was ovarian or uterine in origin.