Inhibins A and B are regulated differentially in the early post-partum period

The early postpartum period is characterized endocrinologically by a rapid fall in the levels of oestradiol (E2) and immunoreactive inhibin, and a delayed rise in serum follicle stimulating hormone (FSH). No description is currently available of changes in serum inhibins A and B.

The present study aimed to examine the levels of inhibin A and B during the early weeks of postpartum lactational amenorrhoea, to determine whether there was evidence for differential regulation of the two hormones at a time when no dominant follicle was developing in the ovary.

Serum samples were available from 12 subjects aged 29-38 in whom postpartum levels of FSH, immunoreactive inhibin, E2 and prolactin had been examined previously. Samples for hormone assays had been obtained prior to delivery, daily for the 3-5 days postpartum, and weekly thereafter. Inhibins A and B were measured by specific ELISA assays, and results were calculated as 10 day averages for samples obtained on days 1-10, 11-20, 21-30, etc. postpartum. Normal hormone concentrations for reference were obtained from volunteers, also aged 29-38, sampled on days 3-5 of a normal menstrual cycle.

Inhibin A in the predelivery sample ranged from 62 to 1243 ng/l, geometric mean 592. Concentrations fell rapidly postpartum and reached a nadir in the low follicular phase range between days 5 and 37 postdelivery, mean 13.6 days. The concentrations of both inhibin B and FSH rose 14-27 days postpartum from their initially low postdelivery levels, to reach the normal follicular phase range. These increases in concentration were significantly correlated in all individual subjects.

The secretion of inhibin A and B is regulated differentially during the early stages of lactational amenorrhoea, just as it is in the late luteal phase of the normal menstrual cycle. Whilst inhibin A falls postpartum, reflecting cessation of placental function, and remains low until ovulatory cycles are resumed, FSH and inhibin B rise after a delay of two weeks or more. It is postulated that the rise of inhibin B is the result of secretion from a cohort of small follicles stimulated by rising FSH levels.