What is the Origin of Testosterone Free Fraction?

This concept developed from recognizing the limitations of total testosterone assays; the term distinguishes the active hormone from the protein-bound reservoir. It is a crucial refinement within modern endocrinology protocols.

What is the Mechanism of Testosterone Free Fraction?

This fraction is governed by the Law of Mass Action, dictated by the concentration of testosterone, the concentration of SHBG, and the binding affinity between the two. Physiological factors that increase SHBG, such as hyperthyroidism or estrogen excess, decrease the free fraction even if total testosterone remains normal. Optimizing lifestyle factors to normalize SHBG directly elevates this critical free fraction.

Testosterone Free Fraction

Meaning

The Testosterone Free Fraction represents the fraction of total circulating testosterone that is unbound to proteins, primarily SHBG and albumin, making it immediately available for diffusion into target cells to exert androgenic effects. This unbound portion is the physiologically relevant measure of androgenic potential, as bound testosterone is inert until released. Clinical assessment often focuses on this fraction for diagnosing hypogonadism.