A Free Androgen Index (FAI) Calculator is a health-related tool used to estimate the amount of biologically active testosterone in the body. While a standard blood test can measure "total testosterone," much of that testosterone is bound to a protein called Sex Hormone-Binding Globulin (SHBG) and is not available for use by the body's tissues. The FAI provides a calculated ratio that gives a better indication of the amount of "free" or active testosterone. Healthcare professionals use this index, particularly when investigating conditions like Polycystic Ovary Syndrome (PCOS) in women or hypogonadism in men, as it offers a more nuanced view of a person's androgen status than total testosterone alone.
formula of Free Androgen Index Calculator
The Free Androgen Index is a calculated ratio based on two key blood test results. For the calculation to be accurate, both laboratory values must be in the same units, typically nanomoles per liter (nmol/L).
Free Androgen Index (FAI)
This is the primary formula used to calculate the index.
Formula:
Free Androgen Index (FAI) = (Total Testosterone / Sex Hormone-Binding Globulin) * 100
The result is a unitless ratio.
Required Variables
To use the formula, you need two values from a blood test.
- Total Testosterone
This is the total concentration of the testosterone hormone in your blood.
Variable: Total Testosterone (nmol/L) - Sex Hormone-Binding Globulin (SHBG)
This is the concentration of the SHBG protein, which binds to testosterone.
Variable: Sex Hormone-Binding Globulin (SHBG) (nmol/L)
Unit Conversion Formula
Lab reports can provide values in different units. If your Total Testosterone value is in nanograms per deciliter (ng/dL), you must convert it to nmol/L before using the FAI formula.
Formula:
Total Testosterone (nmol/L) = Total Testosterone (ng/dL) * 0.0347
Typical Free Androgen Index (FAI) Reference Ranges
This table provides general reference ranges for FAI in adults. These ranges can vary slightly between different laboratories, and interpretation should always be done by a qualified healthcare professional.
| Population | Typical FAI Range | Interpretation |
| Adult Males | 30 - 150 | Values below this range may suggest low testosterone activity. |
| Adult Females | 0 - 10 | Values above this range may indicate excess androgen activity (hyperandrogenism). |
Example of Free Androgen Index Calculator
A patient receives her blood test results and wants to understand her Free Androgen Index.
First, she gathers her results from the lab report.
- Total Testosterone: 1.5 nmol/L
- Sex Hormone-Binding Globulin (SHBG): 40 nmol/L
Since both values are already in nmol/L, she can directly use the FAI formula.
Step 1: Divide Total Testosterone by SHBG.
Ratio = 1.5 / 40
Ratio = 0.0375
Step 2: Multiply the result by 100.
FAI = 0.0375 * 100
FAI = 3.75
Therefore, her Free Androgen Index is 3.75. According to the reference table, this value falls within the typical range for an adult female.
Most Common FAQs
Total testosterone measures all the testosterone in your blood, but a large portion of it is tightly bound to the SHBG protein, making it inactive. The FAI provides an estimate of the "free" portion that is biologically active and available to your body's cells. Two people could have the same total testosterone level, but if one has much higher SHBG, they will have less active testosterone and a lower FAI, which is clinically more significant.
In women, a high FAI is often associate with conditions of excess androgen, such as Polycystic Ovary Syndrome (PCS), and can be linked to symptoms like hirsutism (excess hair growth) and acne. In men, a low FAI can be an indicator of hypogonadism (low testosterone), which may cause symptoms like fatigue, low libido, and decreased muscle mass. However, the FAI is just one piece of the puzzle, and a diagnosis should only be made by a doctor.
No, the FAI is an index, or an estimation, not a direct measurement. It provides a useful and widely available clinical indicator of androgen status. More complex and expensive laboratory tests, such as equilibrium dialysis, can measure free testosterone directly, but the FAI is often use as a reliable and accessible first step in the evaluation process.