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Androgenfaiserplsrto

Published: August 9, 2026Updated: August 15, 2026

The Free Androgen Index is a blood test that measures the amount of active testosterone available in your body by calculating the ratio of testosterone to a protein called SHBG.

Quick summary

{"Measures active testosterone levels in the blood","Helps diagnose conditions like PCOS, hypogonadism, and androgen excess","Requires a simple blood draw, usually in the morning when testosterone levels are highest","Results are calculated as a ratio and interpreted based on age, sex, and clinical symptoms","Abnormal results may indicate hormonal imbalances requiring further evaluation"}

Medically reviewed by WellAlly Editorial TeamLast reviewed August 15, 2026Editorial policy

Key facts at a glance

LOINC
24125-7
Short code
FAI
Synonyms
Androgen FAI SerPl-sRto
Category
chemistry
Sample
serum
Container
Red-top serum separator tube (SST) or plain red-top tube
Units
N/A
Ref low
0.5
Ref high
150
Fasting
false
Population
adult
Sex
all
Age range
18–99 years
Method
Calculation (Free Androgen Index = [Total Testosterone / Sex Hormone Binding Globulin] x 100)
CPT
84403 (Testosterone) + 84270 (SHBG); calculated index may not have dedicated CPT
SNOMED
442702008
Specimen volume
2 mL serum (minimum 1 mL)
Stability
Serum stable for 7 days at 2-8°C; for longer storage, freeze at -20°C or below for up to 6 months
Hemolysis max
50
Lipemia max
100
Icterus max
40
TAT
4–24 hours
Order
routine

What is this test?

The Free Androgen Index (FAI) is a calculated value that estimates the amount of bioavailable testosterone in your body. Testosterone is a hormone found in both men and women, though in different amounts. Most testosterone in the blood is bound to proteins—primarily Sex Hormone-Binding Globulin (SHBG)—and only the 'free' or unbound testosterone is biologically active. The FAI helps determine how much testosterone is actually available for your body to use.

How is the test performed?

A healthcare professional will draw a small blood sample from a vein in your arm. The sample is then sent to a laboratory where measurements of total testosterone and SHBG are taken. The FAI is calculated using the formula: (Total Testosterone ÷ SHBG) × 100. The test is typically performed in the morning, as testosterone levels follow a daily rhythm and are highest at that time.

Why might I need this test?

Your doctor may order an FAI test if you have symptoms suggesting abnormal testosterone levels. In women, this may include irregular periods, excessive hair growth (hirsutism), acne, or difficulty becoming pregnant. In men, symptoms may include low libido, erectile dysfunction, fatigue, loss of muscle mass, or mood changes. The test helps distinguish between total testosterone levels and the biologically active form.

How should I prepare?

Follow your healthcare provider's instructions carefully. You may need to fast for 8-12 hours before the test. Certain medications can affect test results, so inform your doctor about all medications, supplements, or herbal remedies you are taking. Avoid strenuous exercise and stress before the test, as these can temporarily affect hormone levels. Timing matters—hormone levels fluctuate throughout the day, so the test is usually scheduled in the morning.

Reference range

Adult Males: 30-150; Adult Females: 0.5-8.5; values vary by assay and laboratoryN/A

Specimen: serum

No special preparation required. Sample should be collected in the morning (7-10 AM) for optimal diurnal consistency, especially in younger patients.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Frequently asked questions

What do abnormal FAI results mean?
Abnormal FAI results depend on your sex and symptoms. In women, high FAI may suggest PCOS, excess androgen production, or insulin resistance. Low FAI in women is less common but may indicate low androgen production. In men, low FAI often suggests hypogonadism or problems with testosterone production, while high FAI may require further investigation for underlying conditions. Your doctor will interpret results in context of your symptoms, other test results, and medical history.
Are there factors that can affect my FAI results?
Yes, several factors can influence FAI results. Age is important—SHBG levels increase with age, which can affect FAI calculations. Certain medications such as birth control pills, corticosteroids, and anticonvulsants can change SHBG levels. Liver disease, thyroid disorders, and obesity can also affect results. Women naturally experience fluctuations during menstrual cycles. It's important to discuss any relevant health conditions or medications with your healthcare provider before the test.
Do I need any other tests along with FAI?
Often, yes. Your doctor may order additional tests to get a complete picture of your hormonal health. These commonly include total testosterone, SHBG, estrogen, luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and DHEA-S. In women, a pelvic ultrasound may be recommended if PCOS is suspected. The combination of tests helps your doctor make an accurate diagnosis and determine the best treatment approach.
Is the FAI test safe?
The FAI test is very safe. It involves a standard blood draw, which may cause minor bruising, soreness, or dizziness in some people. These side effects are usually temporary and resolve quickly. The amount of blood taken is small and does not pose any significant health risks. If you have a bleeding disorder or take blood thinners, inform your healthcare provider before the test.
  1. 1
    MedlinePlus - TestosteroneSource
  2. 2
    Mayo Clinic Laboratories - Free Androgen IndexSource
  3. 3
    UpToDate - Evaluation of hyperandrogenism in womenSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy