Free Androgen Index (FAI)

Specialist

also —

What it isA calculated estimate of free testosterone derived from total testosterone divided by SHBG and multiplied by 100.

DoesApproximates the free testosterone fraction when direct measurement isn't available.

GoodA high FAI in men signals strong functional androgen action.

BadA low FAI in men signals functional hypogonadism. In women, a high FAI signals androgen excess (PCOS, hirsutism).

Standard

Men >35%; women <8%.

Optimal

55–90% men, 1–4% women (narrow target)

Modifiers

SHBG dominates the calculation.

How to optimize

within Men's Hormones