ratio estimating active testosterone in the bloodstream
The Free Androgen Index (FAI) is a ratio used to estimate the amount of active, or "free," testosterone in the bloodstream. It is calculated by dividing the total testosterone level by the sex hormone-binding globulin (SHBG) level and then multiplying by 100. This index is particularly useful in diagnosing conditions related to hormonal imbalances, such as polycystic ovary syndrome (PCOS) and certain types of alopecia.