| Inhibins are heterodimeric proteins that suppress the secretion of FSH (follicle stimulating hormone) from the pituitary. Inhibin consists of two distinct chains, or subunits (alpha and beta), linked together. Inhibin A consists of the alpha-subunit and betaA-subunit. Inhibin B consists of the alpha-subunit and betaB-subunit. Only the dimeric forms of the molecule, containing both the alpha and beta subunits, are bioactive, although the free subunit forms exist in circulation.
Inhibins are secreted by ovarian granulosa cells in females and by testicular Sertoli cells in males [1]. Both inhibin A and inhibin B are produced in females, but in males inhibin B is the major circulating inhibin.
During the menstrual cycle and early pregnancy, inhibin A is produced by the corpus luteum [1]. At the onset of menstruation during the early follicular phase, very low levels of inhibin A are found [2]. Levels increase dramatically in the late follicular phase and maximize in the mid-luteal phase [2]. During pregnancy, inhibin A levels in maternal serum are considerably higher. Maternal serum levels of inhibin A increase during the first trimester and decline after about 10 weeks. Levels remain stable at 15 to 25 weeks and then increase, reaching peak at term [1]. In pregnancy, inhibin A production occurs at a number of sites, including the fetus, placental and fetal membranes, and the ovary [3]. The feto-placental unit appears to be the major source of increased circulating concentrations of inhibin A in early pregnancy [4]. Although the function of inhibin A in pregnancy is unknown, inhibin A may be involved in fetal and placental development [3].
- Wald NJ, et al. Prenat Diag 16: 143-153, 1996.
- Groome N, et al. Clin Endocrinol 40: 717-723, 1994.
- Riley SC, et al. Hum Reprod 11: 2772-2776, 1996.
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