What is the Origin of Free Androgen Index Calculation?

The FAI calculation was developed in the clinical endocrinology setting as an accessible and cost-effective surrogate marker for directly measured free testosterone, which can be technically challenging and prone to laboratory variability. The index's origin lies in the fundamental understanding of steroid hormone transport and the high-affinity binding capacity of SHBG in the systemic circulation.

What is the Mechanism of Free Androgen Index Calculation?

The index is calculated using the formula: (Total Testosterone / SHBG) × 100, or a similar proportional relationship. SHBG acts as a major reservoir for androgens, effectively sequestering them and preventing them from binding to cellular receptors. By mathematically accounting for SHBG levels, the FAI provides an indirect, functional measure of the bioavailable hormone, offering a clearer picture of androgenic activity at the cellular level and guiding precise clinical decision-making.

Free Androgen Index Calculation

Meaning

The Free Androgen Index (FAI) calculation is a clinical tool used to estimate the biologically active fraction of circulating androgens, primarily testosterone, by determining the molar ratio of total testosterone to Sex Hormone-Binding Globulin (SHBG). This calculated index provides a more accessible and often sufficiently accurate representation of the amount of unbound, or ‘free,’ testosterone available to interact with target tissues. It is a critical metric in diagnosing both androgen excess states, such as Polycystic Ovary Syndrome (PCOS), and deficiency states like hypogonadism.