Free Androgen Index
| Clinical | Optimal | |
|---|---|---|
| Male | 24 - 104 | 24–104 |
| Female | 0.5 - 6.9 | <5.0 (varies by laboratory) |
What is Free Androgen Index?
The Free Androgen Index (FAI) is a calculated ratio that provides an estimate of the biologically active (free) testosterone circulating in the blood. It is derived by dividing total testosterone by sex hormone-binding globulin (SHBG) and multiplying by 100. This calculation accounts for the fact that most testosterone in the blood is bound to SHBG and albumin, rendering it inactive.
Only free (unbound) testosterone can enter cells and exert its biological effects. While direct measurement of free testosterone is possible, it is technically challenging and not routinely available. The FAI provides a practical and widely used alternative that correlates well with directly measured free testosterone in most clinical situations.
FAI is particularly useful in women, where it is a key diagnostic tool for polycystic ovary syndrome (PCOS) and other conditions associated with androgen excess. In men, FAI is less commonly used because total testosterone and direct free testosterone measurements are preferred.
Why Free Androgen Index Matters for Your Health
FAI is valuable because total testosterone alone can be misleading. SHBG levels vary significantly between individuals and are influenced by factors including body weight, insulin resistance, thyroid function, liver health, and oestrogen levels. A person with normal total testosterone but low SHBG may have an elevated FAI, indicating excessive free androgen activity.
For women, elevated FAI is one of the Rotterdam criteria for diagnosing PCOS and can explain symptoms such as acne, excess hair growth (hirsutism), hair thinning, and menstrual irregularity. It provides a more complete picture of androgen status than total testosterone alone.
For men, a low FAI may help explain symptoms of testosterone deficiency even when total testosterone appears borderline. It is particularly relevant in conditions that raise SHBG, such as ageing, liver disease, and hyperthyroidism, where total testosterone may be normal but biologically available testosterone is reduced.
Free Androgen Index& Your Wearable Data
The Free Androgen Index (FAI) is a calculated ratio of total testosterone to SHBG, providing an estimate of bioavailable androgen activity. In men, it correlates with free testosterone and reflects androgenic drive. In women, elevated FAI is a key diagnostic feature of polycystic ovary syndrome (PCOS). Wearable data helps contextualise FAI by reflecting its downstream physiological effects.
In men, low FAI manifests as reduced exercise performance, slower muscle recovery, and declining strength — all trackable through wearable fitness data. Wearable metrics showing progressive decline in strength-related activities, increasing exercise heart rates, and poor recovery may indicate falling androgen availability that the FAI captures. This pattern should prompt hormonal investigation.
In women with suspected PCOS, wearable data provides useful supporting information. Cycle irregularity tracked by wearable menstrual cycle features, combined with metabolic indicators like elevated resting heart rate and reduced exercise capacity, may correlate with elevated FAI. Monitoring wearable metabolic metrics alongside hormonal blood tests supports comprehensive PCOS management.
What High Free Androgen Index May Suggest
In women, a high FAI (above 5.0) may suggest PCOS, congenital adrenal hyperplasia, androgen-secreting tumours (rare), or Cushing's syndrome. It may also be elevated in obesity and insulin resistance, as these conditions lower SHBG and consequently raise free testosterone availability.
Symptoms associated with high FAI in women include acne (particularly along the jawline), excess facial or body hair, scalp hair thinning, irregular periods, difficulty conceiving, and mood changes.
In men, high FAI is less clinically significant as androgens are expected to be present at higher levels. However, significantly elevated FAI may warrant investigation for exogenous testosterone use, adrenal disorders, or testicular tumours.
What Low Free Androgen Index May Suggest
In men, a low FAI may suggest hypogonadism (low testosterone) and can be associated with fatigue, low mood, reduced libido, erectile difficulties, and decreased muscle mass. Low FAI in men may result from primary testicular failure, pituitary insufficiency, or conditions that elevate SHBG.
In women, low FAI is typically not concerning and may simply reflect healthy androgen levels. Very low FAI alongside symptoms such as extreme fatigue, reduced libido, or poor wellbeing may occasionally warrant further hormonal assessment.
Conditions that raise SHBG — including hyperthyroidism, liver disease, and oestrogen therapy — can lower FAI independently of actual testosterone production.
How to Optimise Your Free Androgen Index
Food
For women with elevated FAI, focus on reducing insulin resistance through a low-glycaemic diet rich in complex carbohydrates, fibre, and lean proteins. Include spearmint tea, which has shown mild anti-androgen effects in studies. Consume cinnamon, which may improve insulin sensitivity. Ensure adequate zinc and magnesium intake from nuts, seeds, and legumes. For men with low FAI, include zinc-rich foods (oysters, beef, pumpkin seeds) and healthy fats (avocados, olive oil, nuts) to support testosterone production. Ensure adequate vitamin D from oily fish, eggs, and fortified foods.
Lifestyle
For elevated FAI in women, maintain a healthy weight, as even a 5–10% reduction in body weight can significantly lower androgens and improve SHBG. Engage in regular exercise, combining resistance training with aerobic activity. Manage stress, as cortisol and androgens share precursor pathways. For low FAI in men, prioritise resistance training, which is the most effective lifestyle intervention for boosting testosterone. Ensure 7–9 hours of quality sleep. Manage stress effectively. Limit alcohol, which raises SHBG and impairs testosterone production. Maintain a healthy body composition.
Supplements
For elevated FAI (women): Inositol (myo-inositol 2g with D-chiro-inositol 50mg twice daily) has strong evidence for improving insulin sensitivity and reducing androgens in PCOS. Spearmint extract may help modulate androgen levels. Berberine has been shown to improve insulin sensitivity and may help lower androgens. For low FAI (men): Ashwagandha (KSM-66) has clinical evidence for supporting healthy testosterone levels. Zinc supplementation (30 mg daily) supports testosterone production. Vitamin D3 supplementation may support testosterone, particularly in those who are deficient.
When to Speak to Your GP
Women should see their GP if FAI is elevated alongside symptoms of androgen excess such as persistent acne, excess hair growth, hair thinning, or irregular periods. If PCOS is suspected, further assessment including pelvic ultrasound and metabolic screening is recommended. Men should seek advice if FAI is low alongside symptoms of testosterone deficiency. If considering fertility, hormonal assessment should be comprehensive and include specialist input.
References
- NICE. Fertility problems: assessment and treatment. CG156. nice.org.uk
- NHS. Polycystic ovary syndrome — Overview. Updated 2024. nhs.uk
- Endocrine Society. Teede HJ, et al. Recommendations from the international evidence-based guideline for PCOS. Hum Reprod. 2018;33(9):1602-1618. pubmed.ncbi.nlm.nih.gov
- BMJ. Balen AH, et al. The management of anovulatory infertility in women with PCOS. Hum Reprod Update. 2016;22(6):687-708. pubmed.ncbi.nlm.nih.gov
Medical Disclaimer— This content is for general educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Omniwo Ltd is a wellness information service and is not a medical device, clinical laboratory, or regulated healthcare provider under MHRA guidelines. The “optimal ranges” presented on this page are based on published clinical guidelines (WHO, NICE, NHS) and peer-reviewed research; they represent functional wellness targets and may differ from standard laboratory reference ranges. Individual results should always be interpreted by a qualified healthcare professional (such as your GP) who understands your full medical history. Do not start, stop, or change any medication or supplement based solely on this information. If you are experiencing symptoms, seek medical attention promptly.