What is the Origin of Free Testosterone Indices?

This term is foundational to clinical endocrinology, particularly in the diagnosis and management of androgen deficiency in both men and women. The development of indices like the Free Androgen Index (FAI) arose from the need to account for variations in SHBG, which significantly impacts the amount of hormone available to target tissues. It is a critical diagnostic tool in hormonal health.

What is the Mechanism of Free Testosterone Indices?

Testosterone circulates primarily bound to SHBG, which renders it biologically inert. The free testosterone fraction is determined by the total testosterone concentration and the binding capacity of SHBG. A high SHBG level, often influenced by estrogen or thyroid status, can sequester a large amount of total testosterone, leading to low free testosterone indices and corresponding symptoms of deficiency, despite a normal total level.

Free Testosterone Indices

Meaning

A set of clinical laboratory values and calculations used to assess the concentration and biological availability of testosterone that is unbound to Sex Hormone-Binding Globulin (SHBG) or albumin. This free fraction is the biologically active hormone that can readily diffuse into cells to exert its effects. These indices provide a more accurate measure of androgenic status than total testosterone alone.