The Free Androgen Index (FAI) is a calculated ratio used in the clinical assessment of androgen status, particularly in situations where changes in sex hormone-binding globulin (SHBG) levels may alter the amount of biologically active testosterone. FAI is calculated by dividing the total testosterone level by the SHBG level, and then often multiplying by a constant (usually 100) to simplify the value. This index provides an estimate of the amount of testosterone that is not bound to SHBG and is therefore free to act on target tissues. The FAI is especially useful in evaluating hormonal balance in individuals with conditions that affect or are affected by androgen levels, such as polycystic ovary syndrome (PCOS) in women, where elevated androgen levels play a key role, or in assessing men suspected of having hypogonadism. While the FAI can offer valuable insights into androgen status, it is important to consider it as part of a broader hormonal assessment, as factors like obesity, thyroid function, and certain medications can influence SHBG levels and thus the FAI. Direct measurement of free testosterone by equilibrium dialysis remains the gold standard for assessing bioavailable testosterone, but FAI can be a useful surrogate when direct measurements are not available.