What Is the Free Androgen Index & Why Is It Tested?

The free androgen index is a simple calculation that estimates how much testosterone in your blood is actually available to act on tissues. It is computed from two routine blood tests, total testosterone and sex hormone-binding globulin (SHBG), and expressed as a ratio of the two.¹ Doctors order it most often to investigate signs of excess androgen activity in women, and the test has become a routine part of evaluating conditions like polycystic ovary syndrome. But the story behind those two numbers, and what the index can and cannot tell you, is more layered than a simple lab printout suggests.

Why Total Testosterone Alone Is Not Enough

Most testosterone circulating in your blood is not doing anything at any given moment. It is bound to proteins, primarily SHBG and albumin, and only a small fraction floats freely. That free fraction is the portion that can enter cells and trigger androgen-driven effects like oil production in skin, hair growth in certain body areas, and changes in muscle and fat distribution. The balance between free and total testosterone depends heavily on how much SHBG your liver is producing at the time.²

This is why total testosterone can be misleading on its own. A woman might have a total testosterone level that looks perfectly normal, but if her SHBG is unusually low, more of that testosterone is unbound and active. She could have symptoms of androgen excess, like acne or unwanted hair growth, despite a “normal” testosterone result. In one study of women with suspected hyperandrogenism, roughly 36% of patients in one clinical group had elevated total testosterone, while close to 90% of that same group had an elevated FAI.¹ The FAI caught androgen excess that total testosterone missed because it accounted for what SHBG was doing.

How the Index Is Calculated

The math is straightforward. FAI equals total testosterone divided by SHBG, multiplied by 100. Both values need to be in compatible units, typically nanomoles per liter. The result is not a concentration; it is a dimensionless ratio that serves as a rough proxy for the percentage of testosterone that is biologically available. No special equipment is needed beyond the two standard blood tests, which makes FAI convenient for clinics that do not have access to more expensive direct free testosterone assays.

That simplicity is both its strength and a limitation. The formula assumes a fixed relationship between SHBG and testosterone binding, and in reality, that relationship is not perfectly fixed. SHBG’s grip on testosterone varies from person to person, affected by genetic variants in the SHBG gene itself. Some polymorphisms alter SHBG’s binding affinity, meaning the same SHBG concentration can correspond to different amounts of truly free testosterone depending on a person’s genetic makeup.³ Free hormone calculations that assume constant binding affinity may, for people carrying certain SHBG variants, give an inaccurate picture.

The Primary Use Case: Evaluating PCOS

The single most common reason a doctor orders FAI is to help evaluate polycystic ovary syndrome. Biochemical hyperandrogenism, meaning measurably high androgen activity, is one of the key diagnostic criteria for PCOS. But total testosterone often does not flag it. FAI and calculated free testosterone have consistently outperformed total testosterone alone in identifying women with the condition.⁴

An updated meta-analysis pooling data from multiple studies found that FAI has a pooled sensitivity of about 81% and a specificity of about 82% for diagnosing PCOS, with a diagnostic odds ratio of 20. The authors concluded it is a reliable biomarker, particularly valuable in primary care settings where ultrasound is not readily available and clinical signs alone are inconclusive.⁵ That is a solid performance for a single blood-derived marker, though it also means roughly one in five cases can be missed or falsely flagged.

One important caveat emerged from research in adolescents. A study comparing teenage girls with PCOS to those who simply had irregular periods due to obesity and insulin resistance found that FAI values were closely matched between the two groups. SHBG was actually lower in the obese, hyperinsulinemic girls without PCOS than in those who had the condition.⁶ The takeaway is that elevated FAI in a young person does not automatically mean PCOS. Insulin resistance and obesity can push FAI up through their own effects on SHBG, mimicking the hormonal profile of PCOS without the ovarian pathology.

Beyond PCOS: Hair Loss and Metabolic Risk

While PCOS evaluation is the headline use, FAI shows up in other clinical contexts too. Female pattern hair loss, also called androgenetic alopecia, is one. A study comparing women with this type of hair thinning to controls found that FAI differed significantly between the groups and appeared to be a better biomarker for predicting hyperandrogenism than testosterone or SHBG measured alone.⁷ For women experiencing diffuse thinning at the crown or a widening part line, FAI can help determine whether androgen activity is a driving factor or whether other causes should be investigated.

There is also growing evidence linking elevated FAI to broader metabolic problems. In women under 50, a higher FAI was associated with metabolic syndrome and elevated markers of inflammation.⁸ This does not mean FAI causes metabolic problems on its own. Instead, the index serves as a barometer: when FAI is elevated, it often reflects the same insulin resistance and excess body fat that drive metabolic syndrome, making it a useful warning signal during routine evaluation.

Does FAI Actually Track With Symptoms?

You might assume a higher FAI automatically means worse symptoms, more body hair, more acne, more hair thinning. The relationship is real but not as tight as you would expect. In women with PCOS aged 18 to 29, researchers found a significant positive correlation between FAI and a clinical score for excess body hair, particularly on the chin, lower abdomen, and thighs.⁹ But in women aged 30 to 40 with the same condition, that correlation faded and was no longer statistically significant. Age appears to loosen the link between the lab number and the visible symptom.

Other researchers have reported no link at all between FAI and hirsutism scores, with the discrepancy likely down to genetic factors influencing how sensitive individual hair follicles are to androgens, as well as lifestyle and dietary differences.¹⁰ Two women can have the same FAI but very different experiences of androgen-driven symptoms. The index measures circulating androgen availability, but tissue-level sensitivity is a separate variable it cannot capture.

What Else Pushes FAI Around

Because FAI is a ratio with SHBG in the denominator, anything that changes SHBG levels will move the index, even if testosterone itself has not changed. Several common conditions and medications do exactly that.

Body weight is one of the biggest influences. In women with PCOS, BMI was significantly and directly related to FAI. Women with a normal BMI had meaningfully lower FAI compared to those who were obese, even after adjusting for other factors.¹¹ Excess body fat drives insulin resistance, which in turn suppresses SHBG production by the liver. The result is a higher FAI that reflects metabolic status as much as ovarian androgen production.

Thyroid function is another significant confound. Thyroid hormones increase the liver’s production of SHBG through a specific signaling pathway involving a transcription factor in liver cells.¹² In practical terms, an overactive thyroid can inflate SHBG and make FAI look deceptively low, while an underactive thyroid can suppress SHBG and artificially raise FAI. One study found that SHBG changes were significant in hyperthyroidism and that women with severe hypothyroidism saw their low SHBG levels normalize once thyroid function was restored.¹³ If thyroid status has not been checked alongside FAI, the index can be misleading.

Oral contraceptive pills and estrogen-containing medications also markedly raise SHBG, which can dramatically lower FAI. This is why doctors need to know about current medications when interpreting the result, and why FAI is often drawn before starting hormonal treatment.

Reference Ranges and Why They Vary

If you look at your lab report expecting a single universal cutoff for FAI, you will be disappointed. Reference ranges differ by laboratory, assay platform, age, and population. One large study in Chinese women established a reference range of roughly 0.7 to 6.4, with the upper cutoff closely matching a cluster analysis threshold of 6.1.¹⁴ That study also found FAI decreases with age, which means a single cutoff applied to all adult women regardless of age bracket oversimplifies things.

Separate work using modern immunoassays found that the strongest predictor of FAI reference ranges in women was BMI, not age, while in men, age was the dominant factor for several androgen parameters including FAI.¹⁵ The meta-analysis on PCOS diagnosis also noted that variability in FAI cutoff values across studies was a significant source of heterogeneity.⁵ All of this means you should interpret your result against the specific range your lab provides and discuss it with your doctor in the context of your age, weight, and other health conditions, not compare raw numbers to a cutoff found online.

FAI in Men: A Different Story

While FAI is widely used in women, its reputation in men is much weaker. In men, SHBG concentrations span a broader range, and the relationship between SHBG and testosterone binding behaves differently at the extremes. A study comparing FAI to calculated free testosterone in men found that the two correlated poorly, and FAI tended to overestimate free testosterone when SHBG was low.¹⁶ Because of this, most guidelines for evaluating male hypogonadism prefer calculated free testosterone or equilibrium dialysis over FAI. If your doctor orders FAI to evaluate low testosterone symptoms in a man, it is worth asking whether a more direct free testosterone method might be more appropriate.

Timing of the blood draw matters more for men than many patients realize. Testosterone in men follows a daily rhythm, peaking in the early morning and declining through the afternoon. In men aged 30 to 40, afternoon levels were 20 to 25% lower than morning levels, and the difference was large enough that some men who looked testosterone-deficient in the afternoon had completely normal levels at every morning measurement.¹⁷ SHBG itself showed much less daily fluctuation, so the swing in FAI over the course of a day comes almost entirely from the testosterone side. Morning blood draws, ideally before noon, are standard practice for this reason.

Assay Accuracy at Low Testosterone Levels

The accuracy of FAI is only as good as the accuracy of the testosterone and SHBG measurements feeding into it. For women, this is a practical concern because female testosterone levels are naturally low, falling into a range where common immunoassay platforms struggle. Research comparing a widely used commercial immunoassay to mass spectrometry found that the immunoassay showed significant positive bias at testosterone concentrations below 100 ng/dL, exactly the range where most female samples fall.¹⁸ Overestimating testosterone at the low end feeds directly into an inflated FAI. Interestingly, one study found that the diagnostic performance of FAI for predicting PCOS was similar regardless of whether testosterone was measured by mass spectrometry or immunoassay, suggesting the ratio partially corrects for assay-level errors.⁴ Still, if borderline results are driving clinical decisions, asking your lab which testosterone method it uses is reasonable.

Genetic Variation in SHBG

Your genes play a surprisingly large role in your SHBG level and, by extension, your FAI. Multiple polymorphisms in the SHBG gene have been robustly linked to circulating SHBG concentrations. A meta-analysis of fertile and infertile men found that specific SHBG variants shifted SHBG levels by several nanomoles per liter, and that measured testosterone concentrations correlated strongly with genetically driven SHBG variation, maintaining stable free testosterone availability across genotypes.¹⁹ In other words, the body compensates for genetically high or low SHBG by adjusting total testosterone production, keeping the free fraction relatively steady. FAI, which does not account for this compensation, could flag someone as abnormal when their biology is actually well regulated. This is one reason clinicians look at the whole clinical picture rather than treating a single FAI number as diagnostic on its own.

How Treatment Changes FAI

Because FAI responds to both testosterone and SHBG, treatments that target either component can shift the index. The most studied scenario is PCOS management. Combined oral contraceptive pills dramatically raise SHBG, which pulls FAI down. A systematic review and meta-analysis comparing metformin to combined oral contraceptive pills found that the pills were significantly more effective at reducing FAI than metformin alone.²⁰ Combining both treatments produced an even greater FAI reduction than either alone.²¹

This means FAI can also serve as a treatment monitoring tool. If you start a medication aimed at addressing androgen excess, a declining FAI over time provides objective evidence that the hormonal environment is moving in the right direction, even before visible symptoms like hair growth or acne improve. Conversely, a stagnant or rising FAI despite treatment can prompt a reassessment of the approach. Weight loss, by improving insulin sensitivity and allowing SHBG to rise, can lower FAI independently of medication, reinforcing why lifestyle measures are often recommended alongside pharmacotherapy.

When to Ask About FAI

Not every hormonal complaint warrants an FAI test, and not every lab panel includes it automatically. If you are experiencing symptoms commonly linked to androgen excess, such as persistent adult acne concentrated along the jawline, new or worsening facial or body hair, thinning hair at the crown of the scalp, or irregular periods, FAI can be a useful addition to the initial workup. It is especially informative if a total testosterone result has already come back “normal” but your symptoms strongly suggest androgen involvement.

For the result to be interpretable, thyroid function should ideally be checked at the same time, and your doctor should know about any hormonal medications you are taking, including birth control pills and hormone replacement therapy. Fasting is not strictly required for FAI, but morning timing is preferred, particularly in men. If you are significantly overweight, an elevated FAI may partly reflect insulin resistance rather than a primary androgen problem, and addressing insulin sensitivity could be a more effective intervention than anti-androgen therapy alone.