Overview
Inhibins are polypeptide hormones that play an essential role in the regulation of reproductive function. They are secreted by Sertoli cells in the testes and by granulosa cells in the ovaries, with additional synthesis occurring in the placenta. The primary physiological function of inhibin is to selectively inhibit the secretion of follicle-stimulating hormone (FSH) from the anterior pituitary, without significantly affecting luteinizing hormone (LH).
Structurally, inhibins are dimeric proteins composed of a common alpha (α) subunit linked to one of two beta (β) subunits. This structural variation gives rise to two biologically active forms: inhibin A (α + βA) and inhibin B (α + βB). In women, inhibin B is predominantly produced by developing ovarian follicles, while inhibin A is mainly secreted by the corpus luteum. In men, inhibin B is the only form secreted, produced by Sertoli cells, and serves as a marker of testicular function.
Structure and Types
Inhibins belong to the transforming growth factor-β (TGF-β) superfamily and are closely related to activins, which have the opposite effect of stimulating FSH secretion. The dimeric structure of inhibin allows it to exert negative feedback control on the hypothalamic–pituitary–gonadal axis.
Inhibin A and inhibin B differ in their beta subunits and physiological roles. Inhibin B reflects follicular development in females and spermatogenic activity in males, whereas inhibin A is more closely associated with luteal function and pregnancy-related processes.
Role in Males
In males, inhibin B is secreted exclusively by Sertoli cells of the testes. It serves as a key marker of Sertoli cell function and spermatogenesis. Serum inhibin B levels show a strong positive correlation with testicular volume and sperm count.
In infertile men, inhibin B levels are typically reduced, while FSH levels are elevated due to loss of negative feedback. The degree of reduction in inhibin B correlates well with the extent of spermatogenic damage, with the lowest levels seen in cases where spermatogenesis is arrested at early stages. Inhibin also plays a role in regulating spermatogonial proliferation within the testes.
Role in Females
In females, inhibin plays a crucial role in regulating ovarian function and the menstrual cycle. Inhibin B is secreted during the follicular phase by developing follicles and reflects ovarian reserve and follicular activity. Inhibin A is produced predominantly by the corpus luteum during the luteal phase and increases significantly after ovulation.
The dynamic changes in inhibin A and B levels across the menstrual cycle help regulate FSH secretion and ensure appropriate follicular selection and ovulation. These hormonal fluctuations are important in fertility assessment and ovulation induction therapy.
Clinical Indications
Inhibin A has several important clinical applications. It is used in the evaluation of hormonal reproductive disorders and plays a role in pregnancy-related screening, particularly in prenatal screening for Down syndrome as part of the maternal serum quadruple test. In oncology, inhibin A is used alongside CA-125 in the evaluation of ovarian tumors.
Inhibin B is used as a marker for granulosa cell tumors and epithelial mucinous ovarian tumors. It is also elevated in adrenocortical adenomas and carcinomas. In prostatic carcinoma, inhibin levels are typically suppressed. In men, inhibin B is used to assess Sertoli cell function and spermatogenesis.
Methods of Estimation and Sample Collection
Inhibin A and B are measured using quantitative chemiluminescent immunoassays or quantitative enzyme-linked immunosorbent assay (ELISA) techniques. These methods provide sensitive and specific measurement of circulating inhibin levels.
For testing, 3.0 mL of blood is collected in a plain red-capped tube, and serum is separated as early as possible. For inhibin A, samples may be collected on any day. For inhibin B in premenopausal women, sample collection is preferred during the follicular phase of the menstrual cycle. In oncology, inhibin expression can also be assessed using immunohistochemistry on paraffin-embedded tissue blocks.
Interpretation of Levels
Low inhibin levels are associated with elevated FSH levels, reduced oocyte quality, and diminished fertility potential in women. Low inhibin B is seen in premature ovarian failure, while levels remain normal in hypogonadotropic hypothalamic amenorrhea. In males, reduced inhibin B reflects spermatogenic damage and infertility.
High inhibin levels are associated with certain tumors. Inhibin B is elevated in granulosa cell tumors and epithelial mucinous ovarian tumors. Inhibin-α shows positivity in a significant proportion of Sertoli cell tumors and in virtually all Leydig cell tumors, aiding tumor differentiation.
Clinical Uses
In males, inhibin testing is used for evaluation of Sertoli cell function and assessment of spermatogenesis. In females, it assists in the assessment of ovarian reserve and serves as a tumor marker for granulosa cell tumors.
In pregnancy, inhibin A is an important component of prenatal screening for Down syndrome when used in combination with AFP, hCG, and unconjugated estriol.
Limitations
Inhibin testing has several limitations. Levels fluctuate during the menstrual cycle, and methodological differences exist between assay platforms. The test lacks specificity for malignancy and should not be used as a standalone diagnostic marker.
Inhibin B is no longer considered a reliable standalone marker for ovarian reserve assessment, and its utility in evaluating male fertility is limited, as it reflects spermatogenesis rather than overall fertility potential. Results must always be interpreted alongside clinical findings and other hormonal investigations.
