FAI - The Free Androgen Index

FAI, a suitable surrogate for Free Testosterone?

Dicken Weatherby, N.D. and Beth Ellen DiLuglio, MS, RDN, LDN

The Free Androgen Index (FAI) has been used as a surrogate for free testosterone and this post shows why this isn't the best way to estimate Free Testosterone when it too can be easily calculated!

The Free Androgen Index (FAI) has been used as a surrogate for free testosterone though it correlates better in women than in men.

It is not recommended to be used in men because men have a gonadotropin feedback system that regulates testosterone (T) production that compensates for changes in SHBG. Women don’t have this feedback control of T production. The FAI may have some value in assessing androgen status in women but may not be accurate in women who have a low SHBG. 

How to calculate the Free Androgen Index?

FAI can be calculated by dividing the total testosterone by sex hormone-binding globulin (SHBG) and multiplying by 100. Both the Total Testosterone and SHBG must be in nmol/L. Here’s the formula:

FAI = 100 X ((Total Testosterone (nmol/L)/SHBG (nmol/L))

Free Androgen Index (Testosterone/SHBG Ratio) Ranges

Age Male Female
20 - 29  30 - 128 0.4 - 8.4
30 - 39 24 - 122 0.4 - 8.4
40 - 49 14 - 126 0.4 - 8.4
Over 49 18 - 82 0.5 - 6.6

Some labs do provide ranges for FAI but it is not recommended as a stand-alone diagnostic tool, especially when you can quickly calculate Free testosterone.

Calculating Free Testosterone

It is preferable to calculate free T using total T, albumin, and SHBG rather than using the FAI. We do this calculation for you on the software and, as such, do not feel we need to use the FAI because Free T is a much more accurate biomarker for unbound testosterone.

You can also calculate Free testosterone using the calculator on this page: http://www.issam.ch/freetesto.htm

Research

Rosner, William et al. “Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement.” The Journal of clinical endocrinology and metabolism vol. 92,2 (2007): 405-13. doi:10.1210/jc.2006-1864

Keevil, Brian G, and Jo Adaway. “Assessment of free testosterone concentration.” The Journal of steroid biochemistry and molecular biology vol. 190 (2019): 207-211. doi:10.1016/j.jsbmb.2019.04.008

Labcorp Free Androgen Index - https://www.labcorp.com/tests/146688/free-androgen-index-fai

Wang, Lilin, and Juan Li. “The value of serum-free androgen index in the diagnosis of polycystic ovary syndrome: A systematic review and meta-analysis.” The journal of obstetrics and gynaecology research, 10.1111/jog.14681. 8 Feb. 2021, doi:10.1111/jog.14681

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