Understanding Inhibin B: What Your Blood Test Result Means
What is inhibin B? A hormone from the testes and ovarian follicles that checks FSH. What the test shows in men and women, and what low or high results mean.
Inhibin B is a hormone made by the testes in men and by small, growing egg follicles in the ovaries in women. Its main job is to tell the pituitary gland to make less follicle-stimulating hormone (FSH). A blood test for inhibin B is used mainly in three situations: to judge sperm production in men with fertility problems, to follow certain ovarian tumours (granulosa cell tumours), and, less often today, as one of several markers of a woman's egg supply. A low value usually points to fewer working sperm-producing cells or fewer follicles; a clearly high value in a woman outside the normal cycle can point to a granulosa cell tumour.
What inhibin B is and what it does
Inhibin B is a glycoprotein hormone. In men it is secreted by the Sertoli cells of the testes, the "nurse" cells that support developing sperm. In women it is secreted by the granulosa and theca cells of ovarian follicles, mainly small (preantral) follicles.
It works as a brake in a feedback loop. FSH from the pituitary stimulates the testes or ovaries; they answer by releasing inhibin B, which in turn holds FSH down. This is why the two are read together: when inhibin B falls, FSH tends to rise. In women approaching menopause, the fall in inhibin B mirrors the rise in FSH.
Inhibin B should not be confused with inhibin A, a related hormone measured in pregnancy as part of screening for Down syndrome and pre-eclampsia. They are different tests with different uses.
Why the test is ordered
- Male infertility. Inhibin B reflects how much sperm the testes are making. Blood levels are positively related to testicular volume and sperm count. It is used together with FSH and a semen analysis, especially when no sperm are found.
- Granulosa cell tumours of the ovary. These tumours often release large amounts of inhibin B, so the test is used at diagnosis and to watch for a return of the tumour after treatment.
- Ovarian reserve (egg supply). Inhibin B falls as the number of follicles declines with age. It was once used to estimate egg supply, but other tests now do this better (see below).
- Boys. Inhibin B is produced throughout childhood, which makes it one of the few markers of testicular function before puberty, for example after surgery for undescended testes.
Typical values
There is no single reference range for inhibin B. Results depend on sex, age, the stage of puberty and, in women, the day of the menstrual cycle, and they also differ between laboratories and assay methods. Inhibin B is usually reported in picograms per millilitre (pg/mL). Use the range printed on your own report.
Examples from published research, to show the scale rather than to judge your own result:
| Group (one study of 324 men) | Average inhibin B |
|---|---|
| Normal sperm count | 132.67 pg/mL |
| No sperm because of a blockage (obstructive azoospermia) | 124.25 pg/mL |
| No sperm because the testes make very few (non-obstructive azoospermia) | 25.22 pg/mL |
In that study a cut-off of 45.9 pg/mL helped to tell the two kinds of azoospermia apart. Cut-offs like this come from single studies and are not universal.
What low inhibin B usually means
In men, low inhibin B together with a high FSH suggests that sperm production in the testes is reduced. This is useful when a semen analysis shows no sperm: a blockage leaves inhibin B in the normal range, while a problem with sperm production lowers it. In boys with undescended testes, inhibin B tends to be lower, most clearly when both testes are affected, although studies do not all agree.
In women, inhibin B is naturally lower as the egg supply shrinks with age. It is one of the earliest hormonal signs of the menopause transition and becomes undetectable about 4 to 5 years before periods stop. A low value outside this context may suggest a reduced ovarian reserve, but it should not be interpreted alone.
What high inhibin B usually means
In a woman who is past menopause, or whose result does not fit her cycle, a high inhibin B can be a sign of a granulosa cell tumour. Inhibin B and anti-Müllerian hormone (AMH) are currently the most accurate blood markers for these tumours, and reviews find no clear reason to prefer one over the other. Inhibin can also be raised by some other ovarian tumours, so a high value is a reason for further assessment, not a diagnosis. Such tumours can produce oestrogen and cause irregular periods or bleeding after menopause. After treatment, a rising inhibin B or AMH can signal a recurrence before symptoms appear, so doctors often measure both during follow-up.
Inhibin B and ovarian reserve: AMH is preferred
The American Society for Reproductive Medicine (ASRM) states that AMH and the antral follicle count on ultrasound are currently the most sensitive and reliable markers of ovarian reserve. Inhibin B is mentioned as a direct marker of the follicle pool, but combining it with other tests did not predict better than single tests. Even good markers of egg supply predict the number of eggs collected in IVF well, but say little about the chance of natural pregnancy.
Symptoms
Inhibin B itself does not cause symptoms. Any symptoms come from the underlying situation: infertility, irregular or absent periods, menopausal symptoms, or abnormal bleeding with a granulosa cell tumour.
What usually happens next
- Men: inhibin B is read with FSH, a semen analysis and testicular volume; in men with no sperm and a normal inhibin B, AMH and other markers may also be needed.
- Women with fertility questions: AMH, an early-cycle FSH and estradiol, and an ultrasound antral follicle count.
- Suspected or treated granulosa cell tumour: inhibin B and AMH, measured at diagnosis and during follow-up. Results are followed over time rather than judged once.
Frequently asked questions
What is inhibin B? It is a hormone made by the Sertoli cells of the testes and by growing follicles in the ovaries. It keeps FSH in check, and its level reflects how active these cells are.
What does an inhibin B test show in men? It reflects sperm production. Higher levels go with larger testes and higher sperm counts; low levels with a high FSH suggest the testes are producing fewer sperm.
What does low inhibin B mean in women? It usually reflects fewer follicles, as happens naturally with age and before menopause. AMH and an ultrasound follicle count are now preferred for assessing egg supply.
What does high inhibin B mean? In a woman after menopause, or one with abnormal bleeding, it can indicate a granulosa cell tumour and needs further assessment. It is also used to monitor these tumours after surgery.
What is a normal inhibin B level? There is no single normal range. It depends on sex, age, puberty and cycle day, and on the laboratory. Your result should be compared with the range on your report and read with FSH.
Is inhibin B the same as inhibin A? No. Inhibin A is mainly used in pregnancy screening; inhibin B is used for fertility and granulosa cell tumours.
When to see a doctor
An inhibin B result is almost always ordered as part of a specific question, so the best next step is to go through it with the clinician who ordered it, together with your FSH and other results. It is worth discussing with your doctor soon if you have been trying to conceive for a year without success (or six months if you are 35 or older), if your periods have become irregular or stopped, or if you have any vaginal bleeding after menopause. Seek urgent care for heavy bleeding or fainting.
Sources
- Testing and interpreting measures of ovarian reserve: a committee opinion (2020) — American Society for Reproductive Medicine
- Reproductive Hormones and the Menopause Transition (Santoro & Randolph, 2011) — Obstetrics and Gynecology Clinics of North America, via PMC
- Clinical application value of Inhibin B in subfertile men: a study of 324 Chinese men (Kong et al., 2021) — Journal of Clinical Laboratory Analysis, via PMC
- The role of inhibin B and antimüllerian hormone for diagnosis and follow-up of granulosa cell tumors (Geerts et al., 2009) — International Journal of Gynecological Cancer (PubMed)
- Inhibin B in healthy and cryptorchid boys (Esposito et al., 2018) — Italian Journal of Pediatrics, via PMC
- Significance of inhibin in reproductive pathophysiology and current clinical applications (Kumanov et al., 2005) — Reproductive BioMedicine Online (PubMed)
- Inhibins as diagnostic markers in human reproduction (Tsigkou et al., 2008) — Advances in Clinical Chemistry (PubMed)
- Infertility — MedlinePlus, U.S. National Library of Medicine
- Pathogenesis and treatment of adult-type granulosa cell tumor of the ovary (Färkkilä et al., 2017) — Annals of Medicine (PubMed)
Decisions about your health should always be made in consultation with your healthcare provider.