The Free Androgen Index (FAI) is a calculated ratio that estimates the amount of biologically active androgens — primarily testosterone — available to act on target tissues. It is calculated by dividing total testosterone by SHBG and multiplying by 100. A higher FAI indicates more free androgen activity, while a lower FAI suggests androgens are being heavily bound and inactivated by SHBG.

FAI is particularly useful in women, where it helps identify androgen excess conditions such as PCOS, which can cause irregular periods, acne, and excess hair growth. In men, it is sometimes used alongside free testosterone to assess whether symptoms of low testosterone are related to high SHBG rather than genuinely low testosterone production.