Inhibin A Test: Functions, High/Low Levels, Pregnancy Role & Clinical Importance
What Is Inhibin A?
Inhibin A is a glycoprotein hormone that plays an essential role in regulating reproductive function. Its primary job is to control the release of Follicle-Stimulating Hormone (FSH) from the pituitary gland through a negative feedback mechanism. By suppressing FSH, Inhibin A helps maintain normal ovarian activity in women and supports testicular function in men.
In women, Inhibin A levels change throughout the menstrual cycle and rise significantly during pregnancy. In fact, Inhibin A is one of the four markers included in the Second Trimester Screening—also known as the Quadruple Test—which helps estimate the risk of chromosomal abnormalities such as:
- Down syndrome (Trisomy 21)
- Edwards syndrome (Trisomy 18)
Outside pregnancy, Inhibin A levels are useful in evaluating reproductive health, diagnosing certain ovarian tumors, and monitoring treatment outcomes in gynecological conditions.
Where Is Inhibin A Produced in the Body?
In Women (Non-Pregnant)
Inhibin A is mainly produced by granulosa cells in the ovaries. These cells help support follicle growth and hormonal regulation during the menstrual cycle.
During Pregnancy
Large amounts of Inhibin A are produced by the placenta. Levels rise during the second trimester, which is why the hormone is used in prenatal screening.
In Men
Small quantities of Inhibin A are produced by Sertoli cells in the testes. Although men produce much lower levels than women, Inhibin A still contributes to reproductive function by helping regulate FSH and supporting sperm production.
Main Functions and Importance of Inhibin A
1. Regulates FSH Levels
Inhibin A sends feedback signals to the pituitary gland to reduce FSH release.
FSH is required for:
- Follicle development in women
- Sperm production in men
Balanced Inhibin A levels help maintain healthy reproductive cycles.
2. Role in the Menstrual Cycle
Inhibin A is vital for maintaining normal ovarian function. It helps regulate:
- Follicle maturation
- Hormonal balance
- Timing of ovulation
A disruption in Inhibin A can lead to menstrual irregularities or hormonal imbalance.
3. Role During Pregnancy
During pregnancy, Inhibin A supports:
- Placental development
- Regulation of fetal growth
- Hormonal stability
Because levels naturally rise in the second trimester, the hormone becomes a useful marker in prenatal screening.
4. Prenatal Screening (Quad Screen)
The Quadruple Test measures:
- AFP (Alpha-Fetoprotein)
- hCG
- Unconjugated Estriol (uE3)
- Inhibin A
Together, these markers help estimate the risk of:
- Down syndrome
- Edwards syndrome
- Certain fetal development abnormalities
Higher-than-normal Inhibin A values—especially above 2.0 MoM—can indicate an increased risk of Down syndrome.
5. Tumor Marker for Ovarian Cancers
Inhibin A is an important biomarker for:
- Granulosa cell tumors
- Mucinous epithelial ovarian tumors
Clinicians may use Inhibin A levels to:
- Aid diagnosis
- Monitor treatment response
- Assess recurrence after surgery
Causes of Low Inhibin A Levels
1. Normal Finding in Non-Pregnant Women & Men
Low or undetectable levels are considered completely normal outside pregnancy.
2. During Pregnancy
Low levels may indicate:
- Edwards syndrome (Trisomy 18)
- Poor placental function
- Risk of fetal growth restrictions
Doctors interpret Inhibin A levels along with other Quad Screen markers for accurate assessment.
3. Ovarian Failure or Menopause
Inhibin A falls significantly when ovarian function declines. This occurs in:
- Premature ovarian insufficiency
- Menopause
- After chemotherapy affecting ovaries
4. Hormonal Disorders
Hormonal conditions that reduce follicular activity may lower Inhibin A production.
Symptoms of Low Inhibin A Levels
Low Inhibin A itself does not produce symptoms. However, underlying causes may show clinical features.
If Due to Ovarian Insufficiency
- Irregular or absent periods
- Infertility
- Hot flashes
- Vaginal dryness
- Symptoms similar to early menopause
During Pregnancy
The mother usually has no physical symptoms, but ultrasound may show:
- Fetal growth issues
- Structural abnormalities (rare)
Low values typically appear in prenatal screening reports before any symptoms occur.
Causes of High Inhibin A Levels
1. Pregnancy
High Inhibin A, especially >2.0 MoM, may indicate increased risk of:
- Down syndrome (Trisomy 21)
Doctors use this value along with other markers and ultrasound findings before making any conclusions.
2. Multiple Pregnancy
Twins or triplets naturally produce higher levels of Inhibin A and other pregnancy hormones.
3. Ovarian Tumors
Higher levels are particularly associated with:
- Granulosa cell tumors
- Mucinous epithelial ovarian tumors
In these cases, Inhibin A is an important diagnostic and monitoring tool.
4. Rare Gynecological Malignancies
Certain rare ovarian or uterine cancers may also elevate Inhibin A.
Symptoms of High Inhibin A Levels
During Pregnancy
Elevated Inhibin A does not cause physical symptoms in the mother. However, it may be associated with:
- Higher risk of Down syndrome
- Placental dysfunction (in some cases)
If High Due to Tumors
The symptoms may include:
- Pelvic or lower abdominal pain
- Abdominal distension or bloating
- Early satiety
- Irregular or heavy menstrual bleeding
- Detection of a pelvic mass (in advanced cases)
These symptoms depend on the size and location of the tumor.
Reference Ranges
Non-Pregnant Women
Very low or undetectable.
Men
Low levels, similar to non-pregnant women.
Pregnancy (Second Trimester)
Reported as MoM (Multiples of the Median).
General guidelines:
- Normal: Around 1.0 MoM
- High Risk for Down Syndrome: Above 2.0 MoM
Exact values vary by:
- Laboratory
- Gestational age
- Maternal weight
- Ethnicity
Always interpret results with full prenatal screening.
Sample Type
- Blood sample (serum) is required
- No special collection tube is needed
Test Preparation
- No fasting required
- Provide accurate gestational age for MoM calculation
- Inform your doctor about:
- Multiple pregnancies
- IVF conception (adjustments may be required)
- Any hormone medications
- If monitoring tumors, follow the doctor’s timing instructions for repeat tests
When to Consult a Doctor
Seek medical advice if you:
- Receive abnormal Quad Screen results
- Have unexplained irregular menstrual cycles
- Experience pelvic pain or abdominal bloating
- Show symptoms suggestive of ovarian insufficiency
- Have a known ovarian tumor and need follow-up testing
- Are pregnant and concerned about fetal screening results
Immediate consultation is necessary if severe symptoms occur during pregnancy or if a tumor is suspected.
Important Word Explanations
FSH: Hormone that controls ovarian follicle growth and sperm production.
MoM (Multiples of the Median): A standardized value comparing your test result to the median for your gestational age.
Granulosa Cells: Ovarian cells that produce estrogen and Inhibin A.
Quadruple Test: Second-trimester screening test for chromosomal abnormalities.
Chromosomal Abnormalities: Genetic disorders like Down syndrome and Edwards syndrome.
~END~
Great info.