What is FAI (Free Androgen Index)?
The Free Androgen Index (FAI) is a calculated ratio that estimates biologically active testosterone levels by comparing total testosterone to sex hormone-binding globulin (SHBG). It helps diagnose hormonal imbalances like PCOS in women and hypogonadism in men more accurately than testosterone alone.
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Understanding the Free Androgen Index
The Free Androgen Index (FAI) is a calculated value that provides insight into how much biologically active testosterone is circulating in your body. Unlike total testosterone measurements, which show all testosterone in your blood regardless of whether it's available for use, FAI gives a more accurate picture of the testosterone that's actually free to interact with your cells and tissues.
Think of FAI as a way to understand your body's true testosterone activity. While total testosterone tells you how much of the hormone is present, much of it may be bound to proteins and unavailable for use. FAI helps clinicians and patients understand what portion of testosterone is genuinely active and influencing bodily functions.
How FAI is Calculated
The Free Androgen Index is calculated using a simple formula that takes into account two key measurements from your blood test:
Normal FAI Reference Ranges
| Population | Normal FAI Range | Clinical Significance | |
|---|---|---|---|
| Women (Premenopausal) | Women (Premenopausal) | 0.4-8.4 | Higher levels may indicate PCOS or other androgen excess |
| Women (Postmenopausal) | Women (Postmenopausal) | 0.4-6.0 | Levels naturally decline after menopause |
| Men (Adult) | Men (Adult) | 30-150 | Lower levels may indicate hypogonadism |
Reference ranges may vary by laboratory and testing method. Always consult with your healthcare provider for interpretation.
- Total testosterone levels
- Sex hormone-binding globulin (SHBG) levels
The formula is: FAI = (Total Testosterone / SHBG) × 100
SHBG is a protein produced by your liver that binds to sex hormones, including testosterone, making them inactive. When SHBG levels are high, more testosterone is bound and unavailable. When SHBG levels are low, more testosterone remains free and active. This relationship makes FAI a valuable tool for understanding hormonal balance.
Normal FAI Ranges
Normal FAI ranges vary significantly between men and women, and can also differ based on age and laboratory methods. Understanding where your FAI falls within these ranges helps identify potential hormonal imbalances.
It's important to note that these ranges can vary between laboratories and testing methods. Your healthcare provider will interpret your results based on the specific reference ranges used by your testing facility, along with your symptoms and overall health picture.
Why FAI Testing Matters
FAI testing provides crucial information that total testosterone testing alone might miss. This is particularly important because two people with identical total testosterone levels can have vastly different amounts of active testosterone based on their SHBG levels.
Diagnosing PCOS in Women
For women, FAI is especially valuable in diagnosing polycystic ovary syndrome (PCOS), a common hormonal disorder affecting 6-12% of women of reproductive age. Women with PCOS often have elevated FAI levels due to increased testosterone production, decreased SHBG, or both. Symptoms of high FAI in women include irregular periods, excess facial or body hair, acne, and difficulty losing weight.
Assessing Hypogonadism in Men
In men, FAI helps diagnose hypogonadism (low testosterone) more accurately than total testosterone alone. A man might have normal total testosterone but low FAI if his SHBG levels are elevated, leading to symptoms like decreased libido, erectile dysfunction, fatigue, and loss of muscle mass. Regular monitoring of FAI can help optimize hormone health and track treatment effectiveness.
Monitoring Hormone Therapy
FAI is also valuable for monitoring hormone replacement therapy effectiveness. As treatments can affect both testosterone and SHBG levels differently, tracking FAI provides a more complete picture of treatment response than monitoring testosterone alone.
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Factors That Affect FAI Levels
Several factors can influence your FAI levels by affecting either testosterone production or SHBG levels:
- Age: SHBG typically increases with age, potentially lowering FAI
- Weight: Obesity often decreases SHBG, increasing FAI
- Insulin resistance: Can lower SHBG and increase FAI
- Thyroid function: Hyperthyroidism increases SHBG, while hypothyroidism decreases it
- Liver health: Since SHBG is produced in the liver, liver disease can affect levels
- Medications: Birth control pills, steroids, and certain other medications can impact SHBG
- Diet and lifestyle: High-sugar diets and sedentary lifestyles can affect hormone balance
Symptoms of Abnormal FAI
Recognizing the symptoms of abnormal FAI can help you identify when testing might be beneficial. Symptoms vary based on whether FAI is too high or too low, and differ between men and women.
High FAI Symptoms in Women
- Irregular or absent menstrual periods
- Hirsutism (excess facial and body hair)
- Male-pattern baldness or thinning hair
- Severe acne, especially along the jawline
- Deepening of the voice
- Increased muscle mass
- Difficulty conceiving
Low FAI Symptoms in Men
- Decreased libido and sexual function
- Erectile dysfunction
- Fatigue and low energy
- Depression or mood changes
- Loss of muscle mass and strength
- Increased body fat, especially around the midsection
- Decreased bone density
- Hot flashes or night sweats
Testing and Monitoring FAI
FAI testing requires a simple blood draw that measures both total testosterone and SHBG levels. The test is typically performed in the morning when testosterone levels are at their peak, and fasting may be recommended for the most accurate results.
For women, timing of the test during the menstrual cycle can affect results. Many healthcare providers recommend testing during the follicular phase (days 3-5 of the cycle) for consistency. Women on hormonal contraceptives should discuss timing with their provider, as these medications can significantly affect results.
Regular monitoring of FAI can be valuable for tracking hormonal health over time, especially if you're undergoing treatment for a hormonal condition or making lifestyle changes to optimize your hormone levels. Testing every 3-6 months allows you to see trends and adjust interventions as needed.
Optimizing Your FAI Levels
If your FAI levels are outside the optimal range, several strategies can help bring them back into balance. The approach depends on whether your FAI is too high or too low and the underlying causes.
Lifestyle Modifications
- Maintain a healthy weight: Weight loss can increase SHBG and lower FAI in women with PCOS
- Exercise regularly: Both resistance training and cardiovascular exercise can optimize hormone levels
- Manage stress: Chronic stress affects hormone production and balance
- Improve sleep quality: Poor sleep disrupts hormone production
- Eat a balanced diet: Focus on whole foods, adequate protein, and healthy fats
- Limit alcohol: Excessive alcohol can affect both testosterone and SHBG levels
Medical Interventions
When lifestyle changes aren't sufficient, medical treatments may be necessary. For women with high FAI due to PCOS, options include hormonal contraceptives, anti-androgen medications, or insulin-sensitizing drugs. Men with low FAI might benefit from testosterone replacement therapy, though this requires careful monitoring and consideration of risks and benefits.
The Connection Between FAI and Overall Health
FAI levels don't exist in isolation—they're connected to many aspects of your overall health. Abnormal FAI has been linked to increased risk of metabolic syndrome, cardiovascular disease, and type 2 diabetes. In women, elevated FAI is associated with insulin resistance and increased cardiovascular risk factors.
Understanding and optimizing your FAI can therefore have benefits beyond addressing immediate symptoms. It's an important piece of the puzzle for maintaining long-term health and preventing chronic diseases.
Taking Action on Your Hormonal Health
The Free Androgen Index provides valuable insight into your hormonal health that goes beyond simple testosterone testing. By understanding the relationship between total testosterone and SHBG, FAI offers a more complete picture of your body's available androgens and their effects on your health.
Whether you're experiencing symptoms of hormonal imbalance, have been diagnosed with PCOS or hypogonadism, or simply want to optimize your health, FAI testing can provide crucial information. Combined with other hormone tests and a comprehensive health assessment, FAI helps create a roadmap for achieving optimal hormonal balance and overall wellness.
Remember that hormonal health is complex and interconnected with many other aspects of your wellbeing. Working with healthcare providers who understand these connections and can interpret your results in context is essential for developing an effective treatment plan tailored to your unique needs.
References
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- Teede HJ, Misso ML, Costello MF, et al. Recommendations from the international evidence-based guideline for the assessment and management of polycystic ovary syndrome. Hum Reprod. 2018;33(9):1602-1618.[PubMed][DOI]
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744.[PubMed][DOI]
- Goldman AL, Bhasin S, Wu FCW, Krishna M, Matsumoto AM, Jasuja R. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocr Rev. 2017;38(4):302-324.[PubMed][DOI]
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