Free Androgen Index Calculator

Historical Background

The Free Androgen Index (FAI) is a clinical marker used to estimate the level of bioavailable androgens (male hormones like testosterone) in the body. It is primarily used in the diagnosis of androgen-related disorders, especially in conditions like polycystic ovary syndrome (PCOS) or hyperandrogenism. Historically, the measurement of testosterone in the blood only included total testosterone levels, but many of these hormones are bound to proteins, making them inactive. The FAI was developed to account for this by using sex hormone-binding globulin (SHBG) levels to estimate the amount of unbound (free) testosterone.

Calculation Formula

The formula to calculate the Free Androgen Index (FAI) is:

\[ \text{FAI} = \left( \frac{\text{Total Testosterone (nmol/L)}}{\text{SHBG (nmol/L)}} \right) \times 100 \]

Example Calculation

If a patient has a total testosterone level of 20 nmol/L and a SHBG level of 40 nmol/L, the FAI would be calculated as:

\[ \text{FAI} = \left( \frac{20}{40} \right) \times 100 = 50 \]

Importance and Usage Scenarios

The Free Androgen Index is an important diagnostic tool for:

• Assessing hyperandrogenism: Elevated FAI levels can indicate excessive androgen activity, which is linked to conditions like PCOS, hirsutism, or male-pattern baldness in women.
• Hormonal balance evaluation: For men, it helps in evaluating hypogonadism (low testosterone levels) and other hormonal imbalances.
• Monitoring therapy: The FAI can be used to assess the effectiveness of hormone therapies or treatments for androgen-related disorders.

Common FAQs

1. What is SHBG?

   • SHBG (Sex Hormone-Binding Globulin) is a protein that binds to sex hormones like testosterone and estrogen, regulating their availability in the bloodstream.

2. Why not just measure free testosterone directly?

   • Measuring free testosterone can be complex and expensive. The FAI provides an easier and more cost-effective estimate, though it is less accurate than direct methods like equilibrium dialysis.

3. What is a normal FAI range?

   • Normal FAI values vary by sex and age, but in women, values under 4-5% are considered normal, while elevated levels may suggest androgen excess. For men, higher values are typical, with hypogonadism indicated by significantly lower FAI levels.

This Free Androgen Index Calculator helps estimate free testosterone levels, making it useful in clinical and research settings for assessing androgen-related conditions.