Free Androgen Index (FAI) is a calculated ratio that estimates how much testosterone is biologically active (available to tissues) in your body. It’s not a hormone measured directly. Instead, FAI compares total testosterone with sex hormone-binding globulin (SHBG), the main protein that binds and inactivates testosterone in the bloodstream. A higher FAI generally indicates more bioavailable testosterone relative to binding capacity.
Key benefits of Free Androgen Index (FAI) testing
- Reveals how much testosterone is actually active and available in your body.
- Spots androgen imbalances that explain acne, hair loss, or excess body hair.
- Flags PCOS early, helping protect fertility and metabolic health long-term.
- Guides treatment decisions for irregular periods, low libido, or mood changes.
- Tracks whether hormone therapy or lifestyle changes are restoring balance effectively.
- Clarifies symptoms when total testosterone levels alone don't explain what you're experiencing.
- Best interpreted with total testosterone, SHBG, and your clinical symptoms together.
What is Free Androgen Index (FAI)?
The Free Androgen Index is a calculated ratio that estimates how much testosterone is biologically active in your bloodstream. It compares total testosterone to sex hormone-binding globulin (SHBG), the protein that binds and inactivates most circulating testosterone.
A window into hormone availability
Most testosterone travels bound to SHBG, making it unavailable to tissues. Only the small unbound fraction can enter cells and trigger effects like muscle growth, libido, and energy regulation. FAI approximates this free, active portion without requiring specialized lab techniques.
Why the ratio matters
When SHBG levels rise, more testosterone gets locked away, reducing what's available for use. When SHBG falls, more testosterone circulates freely. FAI captures this dynamic balance, offering insight into androgen activity that total testosterone alone can miss.
This makes FAI particularly useful in conditions where SHBG levels shift, such as polycystic ovary syndrome, metabolic syndrome, or thyroid disorders. It reflects the functional androgen status your body actually experiences.
Why is Free Androgen Index (FAI) important?
The Free Androgen Index estimates how much testosterone is biologically active rather than bound to carrier proteins in your blood. It matters because active testosterone drives energy, muscle tone, libido, mood stability, and metabolic health in both sexes, while imbalances ripple through reproductive, cardiovascular, and bone systems. FAI is calculated from total testosterone and sex hormone-binding globulin (SHBG), offering a window into androgen availability when direct free testosterone measurement isn't feasible.
When the signal is too quiet
When FAI falls below the typical range, your tissues may not receive enough androgen stimulation. In men, this can manifest as fatigue, reduced muscle mass, low libido, and mood changes. Women with low FAI may experience similar energy dips, though their baseline androgen levels are naturally much lower than men's.
When the signal is too loud
Elevated FAI suggests excess free testosterone, which in women often signals polycystic ovary syndrome (PCOS), triggering irregular periods, acne, excess hair growth, and insulin resistance. Men rarely see high FAI unless using exogenous androgens or facing certain tumors.
The metabolic and reproductive crossroads
FAI connects hormonal balance to insulin sensitivity, ovarian function, cardiovascular risk, and bone density. Persistent imbalances can accelerate metabolic syndrome, infertility, and long-term cardiovascular disease, making FAI a vital marker for understanding how androgens shape whole-body health across the lifespan.
What do my Free Androgen Index (FAI) results mean?
Low values
Low values usually reflect reduced androgen activity relative to binding protein levels. In men, this often signals low testosterone production or elevated sex hormone binding globulin, which can accompany aging, obesity, metabolic syndrome, or thyroid excess. The result may be reduced energy, libido, muscle maintenance, and mood stability. In women, low FAI is less commonly flagged but may occur with very high binding protein levels or androgen deficiency states.
Optimal values
Being in range suggests balanced androgen bioavailability for your sex and life stage. In men, this supports normal sexual function, muscle and bone health, and metabolic regulation. In women, optimal FAI typically sits in the lower portion of the reference range, reflecting physiologically lower androgen levels that support energy and tissue repair without virilizing effects.
High values
High values usually reflect increased free androgen activity. In women, this is the hallmark of polycystic ovary syndrome, where excess ovarian or adrenal androgen production combines with low binding protein levels. It often accompanies irregular cycles, acne, hirsutism, and insulin resistance. In men, elevated FAI may indicate androgen excess from supplementation, tumors, or conditions that suppress binding proteins like obesity or hypothyroidism.
Notes
FAI is calculated from total testosterone and sex hormone binding globulin, so interpretation depends on both. Pregnancy, oral contraceptives, thyroid disorders, liver disease, and obesity all alter binding protein levels and shift the index independently of true androgen status.
Free Androgen Index is a calculated ratio that estimates how much testosterone is freely available for your cells to use. It uses two blood measurements: total testosterone and sex hormone–binding globulin (SHBG).
Do I need a Free Androgen Index (FAI) test?
Struggling with unexplained weight gain, thinning hair, irregular periods, or stubborn acne? Could your hormone balance be off, and might a Free Androgen Index test reveal what's happening?
The FAI measures the ratio of testosterone to the protein that binds it, showing how much active testosterone is actually available in your body. This matters because even normal total testosterone can mask an imbalance.
Testing your FAI gives you a quick snapshot of your androgen activity, helping pinpoint whether hormonal imbalances are driving your symptoms. It's the essential first step to personalizing your treatment plan and lifestyle adjustments so you can finally address those frustrating changes.
Get tested with Superpower
If you’ve been postponing blood testing for years or feel frustrated by doctor appointments and limited lab panels, you are not alone. Standard healthcare is often reactive, focusing on testing only after symptoms appear or leaving patients in the dark.
Superpower flips that approach. We give you full insight into your body with over 100 biomarkers, personalized action plans, long-term tracking, and answers to your questions, so you can stay ahead of any health issues.
With on-demand access to a care team, CLIA-certified labs, and the option for at-home blood draws, Superpower is designed for people who want clarity, convenience, and real accountability - all in one place.
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Method: Derived from laboratory results. If any input is measured by a laboratory-developed test (LDT) validated under CLIA, that input is not cleared or approved by the FDA. This ratio/index itself is not FDA-cleared. Results support clinician interpretation and are not a stand-alone diagnosis. Inputs: total testosterone, SHBG.
