What is the Origin of Circulating Free Hormone Fractions?

This concept evolved from early hormone assays that measured total hormone levels, leading to clinical confusion regarding functional status. The need to differentiate active hormone from protein-bound reservoirs drove the development of assays focusing on the unbound component. It reflects a refinement in endocrinological measurement science.

What is the Mechanism of Circulating Free Hormone Fractions?

Steroid hormones, being lipophilic, require binding proteins for transport in the aqueous plasma environment. The equilibrium between bound and free hormone is governed by the binding affinity of the carrier protein and the total concentration of the hormone itself. A shift toward increased free fraction signifies enhanced receptor availability and potentially heightened physiological effect at target tissues.

Circulating Free Hormone Fractions ∞ Area

Circulating Free Hormone Fractions

Meaning

The portion of total circulating steroid or thyroid hormones that are unbound to plasma proteins, such as Sex Hormone-Binding Globulin (SHBG) or Thyroxine-Binding Globulin (TBG). Only these unbound fractions possess the necessary physiochemical properties to readily diffuse across cell membranes and interact with intracellular or nuclear receptors to elicit a biological response. Accurate measurement of these fractions is paramount for assessing true hormonal bioavailability.