SERUM INHIBIN DURING LACTATION - RELATION TO THE GONADOTROPINS AND GONADAL-STEROIDS

OBJECTIVE The alms of the study were to describe the changes in serum Immunoreactive inhibin (INH) during normal lactation and to examine th e relations between INH, oestradiol (E(2)) and follicle stimulating ho rmone (FSH), particularly during the first weeks post partum. DESIGN B lood samples were obtained from normally lactating women for hormone m easurements at daily intervals until discharge from hospital, and subs equently at weekly Intervals until the resumption of menses, or one ye ar post partum. SUBJECTS Eighteen breast feeding women aged 27-36 year s volunteered for the study. MEASUREMENTS INH, FSH, luteinizing hormon e (LH), prolactin (PRL), E(2) and progesterone (P-4) were measured by standard radioimmunoassays. Non-linear modelling was used to quantify the hormone patterns observed. RESULTS Hormone levels were compared wi th those found in the follicular phase of the normal menstrual cycle. Levels of INH fell rapidly in the first week post partum and remained at the lower end of the follicular phase range for the period of study , rising only just prior to resumption of menses. E, fell more slowly, into the follicular phase range, reaching the tower end of that range only at about approximately 100 days post partum. FSH levels were sup pressed initially below the follicular phase range, commencing to rise 4.7-24 days post partum, reaching a plateau high in the follicular ph ase range 17.5-53 days post partum, and subsequently showing a slow de cline. Human chorionic gonadotrophin (hCG), initially measured because of its cross-reactivity in the LH assay, fell rapidly post partum and LH remained in the low follicular phase range for several weeks. PRL fell slowly throughout and was still elevated at 150 days post partum, while P-4 fell very rapidly and was less than 1 nmol/l until just pri or to first menses. CONCLUSlONS Inhibin levels fall rapidly post partu m and remain low until close to the time of resumption of follicular a ctivity and menses. The post partum rise in serum FSH appears to be mu ch more closely related to falling oestradiol levels than to the very early and rapid fall in inhibin. Oestradiol thus appears to be the pre dominant negative feedback factor influencing FSH secretion during the post partum period. The low inhibin levels may allow FSH to rise to l evels-high in the follicular phase range under the predominant negativ e feedback control of oestradiol. Inhibin levels do not appear to be a suitable marker of returning fertility.