What is the Origin of Free Androgen Index Assessment?

The index was developed in clinical biochemistry and endocrinology as a practical tool to account for the variability in SHBG, which significantly impacts the amount of free, active hormone. Since direct measurement of Free Testosterone can be technically challenging and expensive, the FAI provides a robust, easily calculable proxy. Its widespread use in clinical practice confirms its utility as a reliable marker of functional androgen activity.

What is the Mechanism of Free Androgen Index Assessment?

The calculation is based on the physiological principle of mass action and binding affinity. Since SHBG binds testosterone with high affinity, a higher SHBG concentration effectively "locks up" more of the total hormone, reducing the free fraction. The FAI reflects this inverse relationship: as SHBG increases, the FAI decreases, signifying less free androgen. The index thus provides an indirect but powerful measure of the androgenic signal available to target tissues.

Free Androgen Index Assessment

Meaning

The Free Androgen Index (FAI) is a calculated clinical ratio used to estimate the biologically active fraction of androgens, primarily testosterone, in the circulation. It is typically calculated by dividing the Total Testosterone concentration by the Sex Hormone-Binding Globulin (SHBG) concentration and multiplying by a constant. This index serves as a pragmatic, cost-effective surrogate for directly measuring Free Testosterone, providing clinicians with a valuable metric for assessing androgen status in both men and women. A low FAI suggests a reduced amount of bioavailable hormone, often correlating with symptoms of androgen deficiency.