What is a Free Androgen Index test?
The Free Androgen Index (FAI) is a calculated value used to estimate the amount of biologically available androgen, particularly testosterone, in the blood.
FAI is calculated using the measured total testosterone and sex hormone-binding globulin (SHBG) levels.
The exact calculation and reporting units may vary depending on the laboratory method.
Because most testosterone in the blood is bound to proteins such as SHBG and albumin, FAI can provide additional information about the amount of testosterone that may be available to tissues.
What is the Free Androgen Index used for?
The FAI may be used to:
Evaluate suspected androgen excess, particularly in females.
Investigate symptoms such as excessive facial or body hair (hirsutism).
Evaluate acne or other signs of increased androgen activity.
Investigate irregular or absent menstrual periods.
Support the evaluation of polycystic ovary syndrome (PCOS).
Assess suspected androgen deficiency in selected clinical situations.
Help interpret testosterone results when SHBG levels are abnormal.
Why do I need a Free Androgen Index test?
Your doctor may request an FAI when symptoms suggest that the amount of biologically available testosterone may be higher or lower than expected.
In females, testing may be considered for:
Excessive facial or body hair
Acne
Irregular or absent menstrual periods
Signs of increased androgen activity
Suspected PCOS
Difficulty becoming pregnant
In males, testosterone and SHBG measurements may sometimes be used to assess androgen status when total testosterone alone does not provide sufficient information.
What happens during the test?
FAI is usually calculated from blood tests for total testosterone and SHBG rather than measured directly as a separate hormone.
A healthcare professional will collect a blood sample from a vein in your arm. The laboratory measures total testosterone and SHBG and then calculates the Free Androgen Index.
Will I need to do anything to prepare?
Usually, no fasting is required, but preparation can depend on the laboratory's testing protocol.
The timing of testosterone testing may be important, particularly in males, because testosterone levels can vary during the day.
FAI interpretation can also be affected by:
Age
Sex
Menstrual cycle phase
Pregnancy
Hormonal contraceptives or other hormone medications
Changes in SHBG levels
Certain medical conditions
Follow your healthcare provider's instructions regarding the timing of the sample and any medications that may affect the test.
Are there any risks?
The test involves a routine blood sample. You may experience mild discomfort, bruising, or slight bleeding at the needle site.
Serious complications are uncommon.
What do the results mean?
There is no single universal reference range for FAI. The expected range depends on factors such as sex, age, laboratory method, and the reference intervals established by the laboratory.
High Free Androgen Index
A high FAI may indicate increased androgen activity or increased availability of testosterone.
In females, possible causes include:
Polycystic ovary syndrome (PCOS)
Other causes of androgen excess
Certain adrenal or ovarian disorders
Reduced SHBG levels, which can increase the proportion of testosterone that is available
A high FAI does not by itself establish the cause of androgen excess.
Normal Free Androgen Index
A normal FAI generally indicates that the calculated androgen availability is within the laboratory's expected range.
However, a normal result does not completely exclude an androgen-related disorder. Results should be interpreted together with symptoms, physical findings, total testosterone, SHBG, and other relevant hormone tests.
Low Free Androgen Index
A low FAI may indicate reduced androgen availability.
Possible contributing factors include:
Increased SHBG levels
Reduced testosterone production
Certain endocrine or systemic conditions
Hormonal medications
The significance of a low result depends on the patient's clinical situation and other laboratory findings.
Important note about the results
The Free Androgen Index is a calculated estimate, not a direct measurement of free testosterone.
FAI is particularly influenced by the SHBG concentration. Therefore, conditions or medications that substantially change SHBG can affect the FAI even when total testosterone remains relatively unchanged.
When accurate assessment of free testosterone is clinically important, your healthcare provider may consider a reliable direct or calculated free testosterone measurement, depending on the available laboratory method.
FAI should always be interpreted together with total testosterone, SHBG, symptoms, age, sex, and other relevant hormone results. Reference ranges and reporting units vary between laboratories.
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