What is the Origin of Free Hormone Fraction?

This is a fundamental concept in endocrinology, developed through the understanding of protein-ligand binding dynamics and the transport of lipophilic hormones in aqueous biological systems. "Free Hormone" distinguishes the unbound, active form from the bound, inactive reservoir, and "Fraction" denotes its quantitative representation as a percentage of the total hormone. It is a critical metric in clinical practice and diagnostics.

What is the Mechanism of Free Hormone Fraction?

The mechanism is based on the principle of mass action and the high-affinity binding of hormones to transport proteins synthesized primarily by the liver. Carrier proteins, like SHBG, act as a circulating reservoir and buffer system, protecting the hormone from rapid metabolism and ensuring a steady, albeit small, release of the free fraction. Changes in the concentration of these binding proteins, often influenced by thyroid status or insulin, directly impact the size of the active free hormone fraction.

Free Hormone Fraction ∞ Area

Free Hormone Fraction

Meaning

The small percentage of a steroid or thyroid hormone circulating in the bloodstream that is not bound to carrier proteins, such as Sex Hormone-Binding Globulin (SHBG) or albumin. This unbound portion is the biologically active component, as only the free hormone is capable of diffusing across cell membranes to interact with intracellular receptors and elicit a physiological response. The concentration of this fraction is a far more accurate measure of a patient’s true hormonal status than total hormone levels.