Inhibin AEdit
Inhibin A is a peptide hormone that sits at a crossroads of reproductive biology and clinical medicine. It is part of the inhibin/activin protein family and functions primarily as a regulator of the hypothalamic-pituitary-gonadal axis. Inhibin A is produced by both the ovaries and the placenta, and it circulates in the bloodstream in conjunction with other hormones to influence follicle development, ovulation, and pregnancy maintenance. The hormone is a heterodimer composed of alpha (INHA) and beta-A (INHBA) subunits, encoded by the INHA and INHBA genes, and it is one of several biologically active forms of inhibin that interact with activin pathways and their binding partners in the gonad and beyond. inhibin activin FSH hypothalamus pituitary gland ovary placenta
Although often discussed in the context of pregnancy, Inhibin A has a defined role in normal female reproduction as well. It provides negative feedback to the anterior pituitary to help regulate the secretion of follicle-stimulating hormone (FSH), thereby modulating follicular development and the timing of ovulation. Inhibin A works in concert with other hormones such as luteinizing hormone (LH), estradiol, and inhibin B to shape the ovarian cycle. In pregnancy, placental production of Inhibin A becomes an additional source of circulating hormone, contributing to its utility as a biomarker used in maternal serum screening programs. FSH gonadotropins placenta ovary granulosa cell pregnancy Down syndrome quadruple test
Structure and biosynthesis
Molecular structure
Inhibin A is a heterodimer formed by the pairing of an alpha subunit with a beta-A subunit, encoded by INHA and INHBA, respectively. The alpha subunit is common to all inhibins, while the beta subunits confer subtype specificity; together they create the active Inhibin A molecule. The broader inhibin family includes other dimers such as inhibin B (alpha plus beta-B) and inhibin C (alpha plus beta-C), which together participate in a network that balances activin signaling and its wide-ranging effects on cell growth, differentiation, and reproductive function. INHA INHBA inhibin activin follistatins
Tissue distribution and biosynthesis
Inhibin A is produced primarily by ovarian granulosa cells in follicles and by placental cells during pregnancy. The balance of production between ovarian and placental sources shifts with physiological state (non-pregnant vs. pregnant) and with age and ovarian reserve. Inhibin A sits alongside other regulators of the reproductive axis, including inhibin B and activins, all of which can be modulated by gonadotropins, sex steroids, and local ovarian environments. granulosa cell placenta ovary pregnancy hormone
Physiological roles
Regulation of the gonadotropin axis
The definitive role of Inhibin A is negative feedback on the anterior pituitary's secretion of FSH. By reducing FSH levels, Inhibin A helps regulate follicular recruitment and growth, contributing to the orderly progression of the menstrual cycle. This feedback mechanism is part of a broader endocrine system that includes the hypothalamus, pituitary, and gonads, with additional modulation from activin signaling, gonadotropin-releasing hormone (GnRH), and circulating sex steroids. FSH hypothalamus pituitary gland GnRH activin
Ovarian function and folliculogenesis
Within the ovary, inhibin A participates in coordinating the maturation of follicles and the luteal phase that follows ovulation. Its activity complements other ovarian hormones to ensure that follicular development proceeds in a controlled fashion and that endocrine signals align with reproductive timing. The interplay among inhibins, activins, and follistatins shapes folliculogenesis and follicle quality across the cycle. ovary granulosa cell folliculogenesis activin follistatins
Pregnancy and placental biology
During pregnancy, placental production of Inhibin A adds a distinct, gestational layer to the hormone’s profile in maternal serum. Placental Inhibin A participates in the broader endocrine environment of pregnancy and is a component of screening strategies used to assess fetal well-being and risk for certain chromosomal conditions. The placental source reflects the placenta’s role as an endocrine organ in maintaining pregnancy and supporting fetal development. placenta pregnancy Down syndrome quadruple test
Clinical significance
Prenatal screening and maternal serum testing
Inhibin A is a key component of certain maternal serum screening panels for fetal aneuploidy, most notably as part of the quadruple test that also includes human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and alpha-fetoprotein (AFP). In many cases, elevated Inhibin A levels in the second trimester, in combination with the other markers, raise the estimated risk for trisomy 21 (Down syndrome) in the fetus. The interpretation of Inhibin A levels depends on gestational age, assay type, and the clinical context, and it is always considered alongside the other markers and maternal factors. Down syndrome quadruple test hCG uE3 AFP pregnancy
Tumor markers and gonadal pathology
Inhibin A has clinical relevance as a tumor marker, particularly for certain gonadal neoplasms. Granulosa cell tumors of the ovary, which arise from sex cord-stromal tissue, can secrete Inhibin A and Inhibin B, making these hormones useful in diagnosis and monitoring of disease. Elevated Inhibin A in a patient with relevant ovarian pathology can inform clinical suspicion and management, and serial measurements may aid in tracking treatment response or recurrence. Placental Inhibin A can also be elevated in certain pregnancy-related conditions, underscoring its broader diagnostic value. granulosa cell tumor ovary placenta
Fertility and reproductive aging
Beyond its immediate role in the cycle, Inhibin A participates in the broader regulation of reproductive aging and ovarian reserve through its relationship with inhibin B and activin signaling. Clinically, measurement of inhibin family members can contribute to assessments of ovarian reserve in some settings, although interpretations require careful integration with age, cycle history, and other hormonal markers. ovary inhibin B activin
Controversies and debates
Prenatal screening and the use of maternal serum markers like Inhibin A intersect with ongoing debates about medical ethics, public health policy, and personal choice. Proponents argue that biomarker-based screening is a rational, data-driven approach to inform expectant parents and healthcare providers, enabling informed decisions and planning. Critics worry about false positives and negatives, potential anxiety, and the broader social implications of screening programs, including how information is used and who bears the downstream costs and decisions. From a more conservative policy perspective, the emphasis is often on patient autonomy, informed consent, and minimal government intrusion, with a focus on ensuring that testing remains voluntary, private, and proportionate to the clinical context. Advocates for evidence-based obstetrics tend to stress that screening, when well-validated and properly implemented, improves outcomes by enabling timely diagnosis and management while avoiding unnecessary interventions. Critics of what they describe as overmedicalization often stress that screening results should not be a coercive determinant of pregnancy decisions and should always be handled with clear communication and support. The dialogue around these issues reflects broader tensions about science, medicine, and the appropriate scope of public health programs, while maintaining a focus on patient-centered care and the responsible use of medical information. Woke criticisms that frame such screenings as inherently coercive or discriminatory are generally countered by notes that screening is a choice, is aimed at information and planning, and is subject to ethical safeguards and informed consent. Down syndrome quadruple test pregnancy