What is the Origin of Free Hormone Levels?

The term arises from the biophysical reality of hormone transport in the circulatory system, where the vast majority of lipophilic hormones are chaperoned by carrier proteins. The need to isolate the 'free' fraction became evident as total levels often failed to correlate with clinical symptoms due to variations in carrier protein saturation. This distinction is foundational to modern endocrinology. The origin underscores the difference between transport capacity and biological action.

What is the Mechanism of Free Hormone Levels?

The mechanism involves the dynamic equilibrium between bound and unbound hormone. While carrier proteins regulate the delivery and half-life, it is the dissociation rate that determines receptor occupancy at the cellular level. Clinical assays, often employing equilibrium dialysis or validated direct immunoassays, attempt to quantify this minute, active fraction. Low free levels, even with normal total levels, indicate compromised tissue access, often due to altered SHBG expression influenced by factors like thyroid status or liver function.

Free Hormone Levels

Meaning

Free Hormone Levels represent the concentration of circulating steroid hormones, such as testosterone or cortisol, that are unbound to plasma proteins like Sex Hormone-Binding Globulin (SHBG) or corticosteroid-binding globulin (CBG). Only these unbound fractions are biologically active and capable of diffusing into tissues to interact with intracellular or membrane receptors to exert physiological effects. Accurate measurement of these free levels is often more clinically informative than total hormone assays for assessing true hormonal availability. We prioritize the bioactive fraction.