inhibinen

Inhibins are glycoprotein hormones of the transforming growth factor-beta (TGF-β) superfamily that act to inhibit follicle-stimulating hormone (FSH) secretion. They are heterodimers composed of an alpha subunit paired with a beta subunit, producing two main forms: inhibin A (alpha–beta-A) and inhibin B (alpha–beta-B). In some languages, the term inhibinen or similar forms is used to refer to these molecules collectively.

Production and structure: Inhibins are primarily secreted by gonadal tissues, with granulosa cells in the ovary and Sertoli cells in the testis as main sources. The alpha subunit is encoded by INHA, while the beta-A and beta-B subunits are encoded by INHBA and INHBB, respectively. Placental tissue also contributes to circulating inhibin A during pregnancy.

Mechanism of action: Inhibin reduces FSH secretion by interfering with activin signaling at the level of the pituitary. It binds to activin type II receptors in the presence of the co-receptor betaglycan (TGF-β type III receptor), inhibiting activin-stimulated transcription that would otherwise promote FSH release. Thus, inhibin provides negative feedback within the hypothalamic–pituitary–gonadal axis.

Physiology and clinical significance: Inhibin B is associated with follicular development and Sertoli cell function, while inhibin A rises during the luteal phase and is markedly elevated in pregnancy. Inhibin levels generally decline with age and diminishing ovarian reserve. Clinically, circulating inhibins can serve as markers of ovarian function and, when elevated, as potential indicators of granulosa cell tumors. They are also used in fertility assessments and differential diagnoses of gonadal disorders.

Measurement: Inhibin concentrations are measured by immunoassays, such as ELISA or immunoradiometric assays, with interpretation influenced by age, sex, menstrual cycle phase, and assay characteristics.