What is the Origin of Serum Free Androgen Index?

The concept emerged from the recognition in clinical endocrinology that total hormone levels can be misleading due to variations in the concentration of transport binding proteins. Since direct measurement of free hormone is often complex and expensive, the index was developed as a standardized, clinically practical calculation. It is a critical tool for diagnosing subtle hypogonadism or androgen excess, particularly in the presence of altered SHBG levels.

What is the Mechanism of Serum Free Androgen Index?

The calculation is based on the equilibrium between bound and unbound hormone: Free Androgen Index (FAI) = (Total Testosterone / SHBG) × 100. SHBG binds to testosterone with high affinity, rendering the bound fraction inactive. A low FAI suggests that a disproportionately high amount of testosterone is sequestered by SHBG, leading to symptoms of androgen deficiency even if total testosterone is within the normal range. Conversely, a high FAI indicates greater bioavailability.

Serum Free Androgen Index ∞ Area

Serum Free Androgen Index

Meaning

A calculated clinical metric used to estimate the concentration of biologically available androgens, primarily testosterone, that are not bound to Sex Hormone Binding Globulin (SHBG) in the bloodstream. This index is derived from the ratio of total testosterone to SHBG concentration, providing a more functional assessment of androgen status than total testosterone alone. It serves as a valuable surrogate marker for the amount of hormone actually able to diffuse into target cells and exert its physiological effects.