The Free Androgen Index (FAI) is a ratio used to estimate the amount of biologically active or free testosterone in the body. Testosterone, the primary male sex hormone, circulates in the blood in two main forms: bound to proteins, such as Sex Hormone-Binding Globulin (SHBG), and unbound or free.
The Free Androgen Index is calculated using the following formula:
FAI = (TT × 100) / SHBG
Where:
- TT (Total Testosterone) is the total amount of testosterone in the blood.
- SHBG (Sex Hormone-Binding Globulin) is a protein that binds to sex hormones, including testosterone.
Free Androgen Index reference values
| Age | Male | Female |
|---|---|---|
| < 29 | 30.0 – 128.0 | 0.4 – 8.4 |
| 30 to 39 | 24.0 – 122.0 | 0.4 – 8.4 |
| 40 to 49 | 14.0 – 126.0 | 0.4 – 8.4 |
| > 49 | 18.0 – 82.0 | 0.4 – 6.6 |
By calculating the Free Androgen Index, one can get an estimate of the proportion of total testosterone that is not bound to proteins and is therefore considered more biologically active.
It's important to note that while the Free Androgen Index provides an estimate, direct measurements of free testosterone levels through laboratory tests are considered more accurate. The Free Androgen Index is used in situations where direct measurement may not be feasible or cost-effective. Interpretation of the FAI should be done in consultation with a healthcare professional, as individual variations and other factors can influence the results.
Based on research by
- Chad Haldeman-Englert, Raymond Turley Jr, Tara Novick Free Androgen Index Health Encyclopedia
- F Scopacasa, M Horowitz, J.M Wishart, H.A Morris, B.E Chatterton, A.G Need The relation between bone density, free androgen index, and estradiol in men 60 to 70 years old Bone, Volume 27, Issue 1, 2000, Pages 145-149