Free Androgen Index (FAI) Calculator
Your Free Androgen Index (FAI):
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Testosterone is a crucial hormone, primarily known for its role in male health, but also vital for women. It influences muscle mass, bone density, mood, energy levels, and sexual function. When assessing testosterone status, simply measuring total testosterone isn't always enough. This is where the Free Androgen Index (FAI) comes into play.
What is Testosterone?
Testosterone is the primary male sex hormone (androgen), though women also produce it in smaller amounts. It's responsible for the development of male secondary sexual characteristics, sperm production, and maintaining libido, muscle strength, and bone health. In women, it contributes to libido, bone strength, and energy.
Why is Total Testosterone Not Always Enough?
Most testosterone in your blood is bound to proteins. Two main proteins bind testosterone:
- Sex Hormone Binding Globulin (SHBG): This protein binds testosterone very tightly, making it largely unavailable for use by your body's cells.
- Albumin: This protein binds testosterone more loosely, allowing it to be more readily available.
The testosterone that is not bound to SHBG (i.e., free testosterone and albumin-bound testosterone) is considered "bioavailable" and can exert its effects on the body. Total testosterone measures all forms, but doesn't tell you how much is actually available.
What is SHBG?
Sex Hormone Binding Globulin (SHBG) is a protein produced mainly by the liver. It binds to sex hormones, including testosterone, dihydrotestosterone (DHT), and estradiol (a form of estrogen). SHBG acts like a transport vehicle, but also regulates the amount of free, biologically active hormone available to tissues. High SHBG levels can mean less free testosterone, even if total testosterone is within the normal range. Conversely, low SHBG can mean more free testosterone.
What is the Free Androgen Index (FAI)?
The Free Androgen Index (FAI) is a calculated ratio that provides an estimate of the amount of biologically active testosterone in your body. It's derived from your total testosterone and SHBG levels. While not a direct measure of free testosterone, FAI is a widely used and cost-effective indicator of androgen status, particularly in men.
How is FAI Calculated?
The formula for FAI is straightforward:
FAI = (Total Testosterone / SHBG) * 100
Both Total Testosterone and SHBG should be measured in the same units, typically nanomoles per liter (nmol/L), for this calculation.
Interpreting Your FAI Results
Reference ranges for FAI can vary slightly between laboratories and populations. However, general guidelines for adult males are often:
- Low FAI: Typically below 30. This may suggest that a significant portion of your total testosterone is bound by SHBG, leading to lower levels of bioavailable testosterone. Symptoms of low testosterone (e.g., fatigue, low libido, erectile dysfunction, mood changes) might be present even with normal total testosterone.
- Normal FAI: Generally between 30 and 100. This range usually indicates healthy levels of bioavailable testosterone.
- High FAI: Above 100. This might suggest higher levels of bioavailable testosterone.
For women, FAI values are typically much lower than in men, and different reference ranges apply. A high FAI in women might be associated with conditions like Polycystic Ovary Syndrome (PCOS).
Factors Affecting Testosterone and SHBG
Several factors can influence your total testosterone and SHBG levels:
- Age: Total testosterone naturally declines with age, while SHBG often increases.
- Obesity: Can lower total testosterone and SHBG.
- Insulin Resistance/Type 2 Diabetes: Often associated with lower SHBG and sometimes lower total testosterone.
- Thyroid Disorders: Both hyperthyroidism and hypothyroidism can affect SHBG levels.
- Liver Disease: Can increase SHBG.
- Medications: Certain drugs (e.g., oral estrogens, some anticonvulsants) can increase SHBG, while others (e.g., anabolic steroids) can decrease it.
- Diet and Lifestyle: Extreme dieting, excessive exercise, or chronic stress can impact hormone levels.
Important Disclaimer
This calculator provides an estimate of your Free Androgen Index based on the numbers you provide. It is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Hormone levels are complex, and their interpretation requires a comprehensive evaluation by a qualified healthcare professional who can consider your full medical history, symptoms, and other relevant tests. Always consult with your doctor before making any decisions about your health or treatment.