Free Androgen Index (FAI) Calculator

The free androgen index ratio between the total testosterone level and the sex hormone binding globulin (SHBG) helps screen for abnormal androgen status.

The FAI is intended to estimate the free testosterone level, the form in which it is able to bind to tissue receptors and exert its hormonal effect.

About the FAI

The testosterone in blood can be found under three forms: tightly bound to SHBG, weakly bound to albumin and only a small percentage (<3% in males and <0.7% in females) unbound. This third form is the free testosterone, the only form capable of binding to tissue receptors to exert its effects, hence why it makes for the best marker of androgen status.

FAI = Total Testosterone (nmol/ L)) x 100 / Sex Hormone-Binding Globulin (SHBG) (nmol/ L)

FAI was found in statistical analysis to be a poor predictor of bioavailable testosterone. FAI should not be used in isolation and is often accompanied by other measured or estimated parameters such as gonadotropin levels.

Because SHBG is present in such large excess in women (10–100:1), free testosterone concentrations are driven primarily by SHBG abundance. In addition, testosterone excess in women lowers SHBG concentrations, which raises the free testosterone concentration and contributes to the strong correlation of 1/SHBG with free testosterone.

Typical FAI healthy values range from 30 to 150 in adult men and 7 to 10 in adult women. In men, values below 30 are cause for concern and could contribute to erectile dysfunction whilst in women, higher values are cause for concern and could contribute to polycystic ovary syndrome and hirsutism.