Inhibin

1. Overview

Inhibin is a glycoprotein hormone that plays a critical role in the regulation of the reproductive system by selectively inhibiting the secretion of follicle-stimulating hormone (FSH) from the anterior pituitary gland. It is part of the hypothalamic-pituitary-gonadal (HPG) axis and contributes to feedback control of gonadotropin secretion. There are two main forms of inhibin: Inhibin A and Inhibin B, which differ in structure, timing of secretion, and physiological roles.

2. Location

Inhibin is produced primarily in the gonads:

• In females: Secreted by the granulosa cells of the ovarian follicles. Inhibin B is dominant in the follicular phase, while Inhibin A is secreted mainly by the corpus luteum during the luteal phase.

• In males: Produced by the Sertoli cells of the testes. Inhibin B is the predominant and functionally significant form.

Smaller amounts of inhibin are also produced in the placenta, pituitary gland, brain, and adrenal cortex, but their physiological roles remain less defined.

3. Structure

Inhibins are glycoprotein heterodimers composed of two subunits:

• α (alpha) subunit: Common to both Inhibin A and B.

• β (beta) subunit: Either βA (in Inhibin A) or βB (in Inhibin B), which determines the specific form.

These subunits are linked by disulfide bonds. Inhibin is part of the transforming growth factor-beta (TGF-β) superfamily, which includes activins and other related hormones that share structural features and signaling pathways.

4. Function

The primary function of inhibin is to inhibit FSH secretion from the anterior pituitary:

• Acts as a negative feedback regulator in the HPG axis.

• Antagonizes the action of activin, which stimulates FSH release.

• Modulates reproductive functions such as gametogenesis and steroidogenesis by regulating FSH levels.

5. Physiological role(s)

Inhibin plays multiple roles in reproductive physiology:

• In females:

  • Inhibin B suppresses FSH during the early follicular phase to regulate follicle recruitment.

  • Inhibin A inhibits FSH in the luteal phase and contributes to the control of the next cycle's follicular development.

  • Works with estradiol and progesterone to fine-tune pituitary feedback.

• In males:

  • Inhibin B provides feedback to reduce FSH secretion, maintaining spermatogenesis within physiological limits.

  • Serves as a marker of Sertoli cell function and testicular activity.

In both sexes, inhibin helps maintain hormonal balance and optimal reproductive capacity by regulating FSH-dependent processes.

6. Clinical Significance

Inhibin levels have diagnostic and prognostic utility in various clinical contexts:

• Ovarian reserve and fertility:

  • Inhibin B is a marker of ovarian follicular activity and is used to assess ovarian reserve in women undergoing fertility evaluation.

  • Low Inhibin B levels suggest diminished ovarian function or approaching menopause.

• Testicular function:

  • In males, serum Inhibin B levels reflect Sertoli cell function and spermatogenic activity.

  • Low levels are associated with male infertility or testicular failure.

• Ovarian tumors:

  • Elevated levels of Inhibin A or B can be markers of granulosa cell tumors or other sex cord-stromal tumors.

  • Used for diagnosis and monitoring of treatment response or recurrence.

• Prenatal screening:

  • Inhibin A is one of the components measured in the quadruple screen during the second trimester of pregnancy.

  • Elevated levels may indicate an increased risk of Down syndrome or other chromosomal abnormalities.