Free Androgen Index (FAI)
FAI estimates how much testosterone is biologically active in the body. Calculated from total testosterone and SHBG, it’s most useful in women for detecting androgen excess that total testosterone alone might miss. Primary use is evaluating PCOS and understanding symptoms like hirsutism, acne, and irregular periods.
The Free Androgen Index (FAI) is a calculated value that estimates the amount of biologically active testosterone in the blood. It’s derived from the ratio of total testosterone to sex hormone binding globulin (SHBG). Because SHBG binds testosterone and makes it inactive, the FAI provides a simple way to estimate how much testosterone is actually available to tissues — particularly useful in women where detecting androgen excess can be challenging.
Why does this matter? In women, total testosterone may appear normal even when androgen excess symptoms are present. This happens because low SHBG (common in PCOS and insulin resistance) leaves more testosterone unbound and active. The FAI captures this — a woman with “normal” total testosterone but low SHBG will have an elevated FAI, revealing the true androgen excess. For evaluating hirsutism, acne, PCOS, and other hyperandrogenic conditions in women, FAI is often more sensitive than total testosterone alone.
Key Benefits of Testing
FAI detects androgen excess that total testosterone might miss. In women with low SHBG, total testosterone can be normal while free (active) testosterone is elevated. FAI accounts for SHBG and reveals these hidden cases of hyperandrogenism.
The test is practical and widely available. Rather than directly measuring free testosterone (which requires specialized assays), FAI uses routine measurements of total testosterone and SHBG to calculate an estimate of androgen activity. Most labs can provide FAI or the components needed to calculate it.
What Does This Test Measure?
The Free Androgen Index is calculated using this formula:
FAI = (Total Testosterone ÷ SHBG) × 100
The result is a ratio that reflects the proportion of testosterone that’s not bound to SHBG and is therefore biologically available. Higher FAI means more active testosterone relative to SHBG; lower FAI means less.
Why the Ratio Matters
Testosterone in blood exists in three forms: bound to SHBG (tightly held, inactive), bound to albumin (loosely held, partially available), and free (unbound, fully active). SHBG-bound testosterone is essentially locked away. When SHBG is low, less testosterone is locked up and more is free to act on tissues. When SHBG is high, more is locked away and less is available.
Total testosterone doesn’t account for this. Two women with identical total testosterone can have very different amounts of active testosterone depending on their SHBG levels. FAI incorporates SHBG into the picture, giving a better estimate of true androgen activity.
FAI vs. Directly Measured Free Testosterone
| Measurement | Method | Best Use |
|---|---|---|
| Total Testosterone | Direct measurement | Initial screening; may miss cases with low SHBG |
| Free Testosterone | Direct (equilibrium dialysis) or calculated | Gold standard for active testosterone; expensive/limited availability for direct measurement |
| Free Androgen Index (FAI) | Calculated from total T and SHBG | Practical estimate; excellent for detecting hyperandrogenism in women |
Important Limitation in Men
FAI correlates well with free testosterone in women but is less reliable in men. At the higher testosterone levels found in men, the relationship between FAI and true free testosterone becomes less linear. For evaluating testosterone status in men, directly measured or calculated free testosterone is preferred over FAI.
Why This Test Matters
Detecting Hidden Androgen Excess in Women
Many women with clinical signs of androgen excess — hirsutism, acne, hair loss — have total testosterone levels that fall within the normal range. This is frustrating for patients who clearly have symptoms but are told their tests are “normal.” The explanation is often low SHBG: with less SHBG to bind testosterone, more is free and active even though the total amount is normal.
| Scenario | Total Testosterone | SHBG | FAI | Clinical Meaning |
|---|---|---|---|---|
| Obvious androgen excess | High | Any | High | Clear hyperandrogenism |
| Hidden androgen excess | Normal | Low | High | FAI reveals the problem |
| Normal androgen status | Normal | Normal | Normal | Androgens not elevated |
| Low androgen activity | Normal/Low | High | Low | May have low androgen symptoms |
Supporting PCOS Diagnosis
Biochemical hyperandrogenism. One of the diagnostic criteria for PCOS is evidence of androgen excess — either clinical (hirsutism, acne) or biochemical (elevated androgens on testing). FAI is often the most sensitive biochemical marker, catching cases where total testosterone is normal but androgen activity is truly elevated.
Reflects the metabolic component. PCOS is associated with insulin resistance, which lowers SHBG. The elevated FAI in PCOS reflects both the androgen excess and the metabolic dysfunction — it’s capturing the whole picture.
Monitoring Treatment Response
Tracks improvement. Treatments for hyperandrogenism — oral contraceptives, anti-androgens, metformin, lifestyle changes — affect both testosterone and SHBG. FAI tracks the net effect on androgen activity. Oral contraceptives raise SHBG significantly, which lowers FAI even if total testosterone doesn’t change much.
Value of Testing When PCOS Is Suspected
Women with irregular periods and signs of androgen excess often struggle to get clear answers. Total testosterone may be normal, leaving both patient and doctor uncertain. Testing FAI (or its components) can confirm biochemical hyperandrogenism when total testosterone doesn’t, validating symptoms and enabling appropriate diagnosis and treatment. For women with suspected PCOS, FAI is one of the most valuable tests for establishing the androgen excess criterion.
Value of Early Testing
Many women notice early signs of androgen excess — a few extra chin hairs, mild acne that persists past adolescence, slightly irregular cycles — but dismiss them as minor annoyances. Testing FAI at this early stage can identify hyperandrogenism before symptoms progress to significant hirsutism or before metabolic complications of PCOS develop. Early diagnosis opens the door to lifestyle interventions and treatments that can prevent worsening symptoms and reduce long-term risks like diabetes and cardiovascular disease. Testing when changes first appear, rather than waiting until they’re severe, provides the most options.
What Can Affect FAI?
Factors That Increase FAI
- Low SHBG conditions:
- Obesity and insulin resistance — high insulin suppresses SHBG
- Polycystic ovary syndrome (PCOS)
- Hypothyroidism
- Nephrotic syndrome
- Cushing syndrome
- Androgen use
- Elevated testosterone:
- PCOS
- Congenital adrenal hyperplasia
- Ovarian or adrenal tumors
Factors That Decrease FAI
- High SHBG conditions:
- Oral contraceptive use — estrogen raises SHBG significantly
- Pregnancy
- Hyperthyroidism
- Liver disease
- Aging
- Estrogen therapy
- Low testosterone production
Oral Contraceptives and FAI
Oral contraceptives dramatically raise SHBG — this is one way they help treat hyperandrogenic symptoms. A woman on oral contraceptives will typically have a low FAI regardless of her underlying androgen status. To assess true baseline androgen levels, testing should ideally be done before starting oral contraceptives or after being off them for several months.
When Should You Get Tested?
Primary Indications
Hirsutism: Excess facial or body hair in a male pattern — FAI helps confirm whether androgen excess is present biochemically.
Persistent acne: Adult acne unresponsive to typical treatments, especially with other androgen signs.
Female pattern hair loss: Hair thinning suggesting androgen-related alopecia.
Menstrual irregularities: Especially when combined with signs of androgen excess, suggesting PCOS.
PCOS evaluation: FAI is one of the most sensitive markers for the biochemical hyperandrogenism criterion.
Infertility workup: When PCOS or hyperandrogenism might be contributing.
When Total Testosterone Is Normal but Symptoms Persist
If you’ve been tested for testosterone and told it’s “normal” but you clearly have hirsutism, acne, or other androgen symptoms, FAI (or separate SHBG testing) may reveal what total testosterone missed.
Monitoring Treatment
Tracking response to oral contraceptives, anti-androgens, or metabolic treatments for PCOS.
Understanding Your Results
Interpreting FAI in Women
| FAI Result | Interpretation | Clinical Context |
|---|---|---|
| Elevated | Increased bioavailable testosterone | Supports hyperandrogenism; consistent with PCOS or other causes |
| Normal | Normal androgen activity | Androgen excess less likely; consider other causes of symptoms |
| Low | Reduced bioavailable testosterone | May be on oral contraceptives or have high SHBG; rarely clinically significant in women |
Elevated FAI
Elevated FAI confirms biochemical hyperandrogenism. In the context of menstrual irregularity and/or clinical signs of androgen excess, this supports PCOS diagnosis. The degree of elevation doesn’t necessarily correlate with symptom severity — some women have significant symptoms with mildly elevated FAI, while others tolerate higher levels with fewer symptoms.
Normal FAI with Symptoms
If FAI is normal but symptoms are present, consider other possibilities: increased end-organ sensitivity to androgens (some women’s hair follicles are more responsive to normal androgen levels), other androgen sources (DHEA-S), or non-androgenic causes of symptoms.
FAI on Oral Contraceptives
FAI will typically be low in women taking oral contraceptives because estrogen raises SHBG substantially. This doesn’t reflect true underlying androgen status. For diagnosis, baseline testing off oral contraceptives is preferred. For monitoring treatment effect, low FAI on therapy indicates the medication is achieving its goal.
What to Do About Abnormal Results
For Elevated FAI
Complete the evaluation. If PCOS is suspected, ensure full diagnostic workup — menstrual history, clinical assessment for hyperandrogenism, and often pelvic ultrasound. Check DHEA-S to assess adrenal contribution. Rule out other causes of androgen excess if indicated.
Address underlying factors. In PCOS, insulin resistance often drives both low SHBG and elevated androgens. Lifestyle modifications (weight loss, exercise, dietary changes) improve insulin sensitivity, raise SHBG, and lower FAI — addressing the root cause.
Consider pharmacological treatment:
- Oral contraceptives: Raise SHBG, suppress ovarian androgen production, dramatically lower FAI
- Anti-androgens (spironolactone): Block androgen effects at the receptor level
- Metformin: Improves insulin sensitivity, can raise SHBG and lower androgens
Monitor response. Repeat FAI testing can track whether treatment is achieving biochemical improvement.
For Low FAI
Low FAI in women is rarely a clinical problem. If it reflects oral contraceptive use, it’s expected. If found unexpectedly, ensure the test was interpreted with correct reference ranges for sex and situation.
Related Health Conditions
Polycystic Ovary Syndrome (PCOS)
PCOS is the most common cause of elevated FAI in women. The combination of increased ovarian androgen production and decreased SHBG (from insulin resistance) results in elevated FAI even when total testosterone is normal. FAI is often the most sensitive biochemical marker for the hyperandrogenism criterion of PCOS.
Hirsutism
Excess hair growth in androgen-dependent areas results from elevated androgens or increased follicle sensitivity. FAI helps determine whether biochemical hyperandrogenism underlies the hirsutism. Treatment targets androgen reduction or receptor blockade, and FAI can track response.
Insulin Resistance
Insulin resistance suppresses SHBG production, increasing FAI even without changes in testosterone production. This is why PCOS, obesity, and metabolic syndrome are associated with elevated FAI. Improving insulin sensitivity (through lifestyle or medication) raises SHBG and normalizes FAI.
Female Pattern Hair Loss
Androgen-related hair thinning in women can occur with elevated FAI. However, some women experience this pattern with normal androgens due to follicle sensitivity. FAI helps determine whether androgen excess contributes and whether anti-androgen therapy might help.
Acne
Persistent adult acne, especially along the jawline and chin, can indicate androgen excess. Elevated FAI supports this connection and may guide treatment toward hormonal approaches (oral contraceptives, spironolactone) rather than topical-only therapy.
Congenital Adrenal Hyperplasia
Non-classic CAH can present similarly to PCOS with elevated androgens and menstrual irregularity. FAI may be elevated. Distinguishing CAH from PCOS requires 17-hydroxyprogesterone testing, as treatment differs.
Related Biomarkers Often Tested Together
Total Testosterone — One of the two components used to calculate FAI.
SHBG — The other component of FAI; also provides independent metabolic information.
Free Testosterone — Direct measurement; alternative to FAI for assessing active testosterone.
DHEA-S — Adrenal androgen; helps identify adrenal contribution to androgen excess.
LH and FSH — Gonadotropin ratio often elevated in PCOS.
Fasting Insulin — Assesses insulin resistance underlying low SHBG.
17-Hydroxyprogesterone — Screens for congenital adrenal hyperplasia.
Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.
Frequently Asked Questions
FAI is a calculated ratio of total testosterone to SHBG that estimates bioavailable testosterone. It reflects how much testosterone is actually available to act on tissues, accounting for the binding effect of SHBG.
Many women with androgen excess symptoms have normal total testosterone because their low SHBG leaves more testosterone free and active. FAI captures this by incorporating SHBG — it can detect hyperandrogenism that total testosterone misses.
FAI is less reliable in men because at male testosterone levels, the correlation with true free testosterone is weaker. Directly measured or calculated free testosterone is preferred for men.
Yes, significantly. Oral contraceptives raise SHBG, which lowers FAI. Testing while on oral contraceptives won’t reflect your baseline androgen status. For diagnosis, test before starting or after several months off oral contraceptives.
Elevated FAI indicates increased bioavailable testosterone — biochemical hyperandrogenism. In the context of symptoms (hirsutism, acne, irregular periods), this supports conditions like PCOS. Further evaluation determines the cause.
If you have results for total testosterone and SHBG in the same units, you can calculate FAI: (Total Testosterone ÷ SHBG) × 100. However, interpretation requires appropriate reference ranges, which vary by lab and sex.
Free testosterone is the actual amount of unbound testosterone. FAI is a ratio that estimates free testosterone. In women, FAI correlates well with free testosterone and is more widely available. Both provide similar clinical information about androgen activity.
References
Key Sources:
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666-3672.
- Azziz R, et al. The androgen excess and PCOS society criteria for polycystic ovary syndrome. Fertil Steril. 2009;91(2):456-488.
- Rosner W, et al. Position statement: utility, limitations, and pitfalls in measuring testosterone. J Clin Endocrinol Metab. 2007;92(2):405-413.
