What is the Origin of Free Testosterone Index?

This index was developed in clinical endocrinology to address the limitations of measuring only total testosterone, particularly in aging males or those with significant SHBG variations. It mathematically models the equilibrium between bound and unbound hormone fractions, offering a standardized way to assess functional androgen status. The calculation often involves established formulae utilizing measured total testosterone and SHBG levels.

What is the Mechanism of Free Testosterone Index?

The calculation mathematically models the unbound fraction based on the dissociation constants between testosterone and its primary binding proteins. By factoring in SHBG, the FTI estimates the amount of hormone free to interact with androgen receptors in muscle, bone, and the central nervous system. Interventions aimed at improving the FTI often focus on reducing SHBG synthesis via improved insulin sensitivity or directly modulating SHBG binding capacity.

Free Testosterone Index

Meaning

The Free Testosterone Index (FTI) is a calculated laboratory value representing the biologically active fraction of total testosterone available to target tissues, independent of direct measurement. This index accounts for the influence of Sex Hormone Binding Globulin (SHBG) and Albumin, which bind the majority of circulating testosterone. Clinically, the FTI provides a more accurate reflection of androgenic potential than total testosterone alone, especially when SHBG levels fluctuate due to age or metabolic status. A robust FTI is necessary for optimal anabolic signaling and vitality.