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Inhibin – Hormone, Function & Clinical Significance

Inhibin is a hormone produced by the gonads that suppresses FSH secretion and plays a key role in reproductive physiology and fertility.

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Things worth knowing about "Inhibin"

Inhibin is a hormone produced by the gonads that suppresses FSH secretion and plays a key role in reproductive physiology and fertility.

What is Inhibin?

Inhibin is a glycoprotein hormone produced primarily by the gonads – in women by the granulosa cells of the ovaries, and in men by the Sertoli cells of the testes. It belongs to the TGF-β superfamily (Transforming Growth Factor beta) and exists in two forms: Inhibin A and Inhibin B. The primary function of inhibin is to suppress the release of follicle-stimulating hormone (FSH) from the pituitary gland.

Biological Function

Inhibin acts through a negative feedback mechanism on the pituitary gland. When inhibin levels in the blood rise, the secretion of FSH is suppressed. This allows inhibin to indirectly regulate the maturation of eggs and sperm and the overall reproductive cycle.

Inhibin B: In women, it is mainly produced during the early follicular phase; in men, it serves as a marker for spermatogenesis.
Inhibin A: Primarily produced during the luteal phase and throughout pregnancy.

Clinical Significance

Fertility and Reproductive Medicine

In women, Inhibin B is an important marker of ovarian reserve, reflecting the quantity and quality of remaining eggs. Low Inhibin B levels may indicate a reduced ovarian reserve or an approaching menopause. In men, low Inhibin B levels suggest impaired sperm production and can be useful in the assessment of male infertility.

Prenatal Screening

Inhibin A is one of the four analytes measured in the Quad Screen test, a prenatal screening test performed during the second trimester of pregnancy. Elevated Inhibin A levels may indicate an increased risk for Down syndrome (Trisomy 21).

Tumor Marker

In oncology, inhibin blood levels are used as a tumor marker for certain ovarian tumors, particularly granulosa cell tumors and mucinous cystadenomas. Elevated inhibin levels can indicate the presence or recurrence of such tumors.

Diagnosis and Measurement

Inhibin is measured through a standard blood test. Both Inhibin A and Inhibin B are quantified using specialized immunoassays. Reference values vary depending on sex, age, and the phase of the menstrual cycle. Results should always be interpreted in the full clinical context, alongside other hormones such as FSH, LH, and AMH (Anti-Mullerian Hormone).

Inhibin and Menopause

As women age and transition into menopause, Inhibin B levels decline significantly. Since Inhibin B normally suppresses FSH, falling inhibin levels lead to a rise in FSH – a classic laboratory finding in the perimenopause. This mechanism explains why FSH is considered a standard marker for menopause.

References

Burger HG. - Inhibin. - Reproductive Biology and Endocrinology, 2004.
Groome NP et al. - Measurement of dimeric inhibin B throughout the human menstrual cycle. - Journal of Clinical Endocrinology & Metabolism, 1996.
World Health Organization (WHO) - Laboratory Manual for the Examination and Processing of Human Semen, 5th edition, 2010.

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