Free androgen indexFAI
A calculated estimate of how much of your testosterone is available to tissues.
The free androgen index is not measured but calculated: total testosterone divided by SHBG, the protein that binds most of it, expressed as a ratio. It estimates how much testosterone is unbound and therefore active.
It matters because total testosterone can mislead when SHBG is unusual. A high SHBG binds more of the hormone, so a normal total can still leave little available, and a low SHBG does the reverse.
In practice the index performs reasonably in women, where it is used in assessing conditions such as PCOS, and less reliably in men, where calculated free testosterone or a direct measurement is generally preferred.