Inhibin B is a protein hormone that acts as a feedback regulator for the production of follicle-stimulating hormone, or FSH. Both men and women produce inhibin B, but in different ways. FSH is produced by the anterior pituitary gland, and stimulates the production of sperm and maturation of eggs.
In women, FSH is produced during the first half of the menstrual cycle. This gears up the maturation of an egg, which will be released during ovulation, in the middle of the cycle. It also spurs the production of the hormone estradiol. Follicular granulosa cells in the ovaries then produce inhibin B so the pituitary gland will know the process has started and stop producing FSH.
As women reach menopause, inhibin B production decreases with the level of eggs that are available for fertilization, or ovarian reserve. The level of inhibin B in women can be used to determine fertility. Women with low levels of inhibin B on the third day of the menstrual cycle may not ovulate regularly.
Inhibin B production in men is by the Sertoli cells. FSH is produced by men to stimulate the production of sperm. Inhibin B regulates the FSH to keep sperm production within normal levels. Male fertility can also be assessed with an inhibin B test. Men with lower levels are often less fertile.
There are also some tumors that secrete inhibin B, so high levels may indicate that they are present. Granulosa cell tumors and epithelial tumors of the ovaries are such tumors. Patients with these forms may have their inhibin levels checked to monitor their condition. Postmenopausal women who have a detectable level of inhibin may have one of the tumors.
Another inhibin, inhibin A, can have the same effects on FSH. It is produced at a different time in the menstrual cycle. Inhibin A is also produced by the placenta during pregnancy. The measurement of inhibin A in the blood, along with the levels of alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol during pregnancy can help determine the risk of down syndrome in the fetus. This is called a quad screen.
There is a possible link between inhibin A and stillbirth. One study found that women who experienced stillbirth had elevated levels of inhibin A during the second or third trimester of pregnancy. The numbers used were found via the quad screen for down syndrome, and those who experienced stillbirth had elevated levels of inhibin A only.