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Free Androgen Index

Does this test have other names?


What is this test?

A free androgen index (FAI) is a ratio figured out after a blood test for testosterone. It's used to see whether you have abnormal androgen levels.

Both men and women make male hormones called androgens, which include testosterone. During puberty, testosterone helps children develop into adults. As you age, levels of this hormone can fall, causing health problems for both genders.

A testosterone test is a blood test that measures total testosterone, free testosterone, and a protein called steroid hormone binding globulin (SHBG). A free androgen index measures testosterone in your blood and compares it with the total amount of testosterone and SHBG in your body. 

Why do I need this test?

You may need an FAI if you show signs of abnormal androgen levels, which differ in women and men.

These hormones aid in the development of sex organs and other gender-linked traits. For example, androgens play a role in the production of the female hormone estrogen. When a woman makes too many androgen hormones, she may develop excess body and facial hair. With too little androgen hormones, a woman may become extremely tired, lose bone mass, or have little interest in sex.

If you are a woman, you may have this test if you have excess hair on your body or face. It's possible that unusual hair growth stems from an ovarian tumor or a condition called polycystic ovarian syndrome (PCOS). Among other things, PCOS causes your ovaries to produce too much testosterone.

Testosterone helps boys develop male traits, and produces facial hair in men. If an adolescent boy isn't developing normally, certain organs might not be producing enough of this hormone. Adult men who don't produce enough testosterone may feel weak, lose muscle strength and mass, develop breasts, or lose interest in sex. 

What other tests might I have along with this test?

Your doctor may also order these tests:

  • Total testosterone

  • Prolactin

  • Thyroid-stimulating hormone

If you are a woman, you may also get these tests:

  • Ferriman-Gallwey scale to measure whether you have an unusual amount of body hair, a condition called hirsutism

  • Total testosterone

  • 24-hour determination of urinary free cortisol for Cushing syndrome, a condition that often causes excessive body hair

  • Dehydroepiandrosterone sulfate, or DHEA-S, a marker for an adrenal source of androgens

  • 17-alpha-hydroxyprogesterone (17-OHP)

  • ACTH stimulation of 17-OHP, if your 17-OHP level is not clearly normal

  • Ovarian ultrasound to determine whether you have an ovarian tumor

  • Adrenal CT scan to check for an adrenal tumor or other abnormalities 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Test results vary by age, gender, and overall health. Normal FAI values generally range between 30 and 150 in men. Levels below 30 indicate a possible testosterone deficiency. Women generally have FAI values of 7 to 10. Normal ranges of serum testosterone for adults are listed below.


  • 300 to 1,000 nanograms per deciliter (ng/dL)

If you are male and your level of total testosterone falls below 200 ng/dL, you may be diagnosed with hypogonadism, an androgen deficiency that causes testosterone levels to drop.


  • 20 to 70 ng/dL

Your hormone levels may be affected by a temporary health condition, such as pregnancy. You may also have inconclusive results and need to be retested. 

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore. 

What might affect my test results?

If you are pregnant, your ovaries may overproduce testosterone as part of a normal pregnancy. Having a tumor or cyst in your ovaries or adrenal gland can also cause your testosterone level to rise.

People with Cushing syndrome may also have higher levels of testosterone.

Certain drugs can affect hormone production, including some steroids and opiates. Let your doctor know about any prescription medicine you take.  

How do I get ready for this test?

You don't need to prepare for this test.  But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.