What is the Origin of Free T Bioavailability?

Free denotes the non-protein-bound state, implying biological activity, while T refers to the thyroid hormones. Bioavailability describes the fraction of administered or endogenously produced hormone that reaches systemic circulation intact. The concept developed as laboratory methods improved to separate bound from unbound fractions, recognizing that binding proteins like TBG regulate access. This distinction is critical in endocrinology for accurate diagnosis. It represents the functional availability of the hormone.

What is the Mechanism of Free T Bioavailability?

The transport of thyroid hormones in blood is largely mediated by binding proteins, predominantly Thyroxine-Binding Globulin (TBG), transthyretin, and albumin. Only the small, unbound fraction can interact with nuclear thyroid hormone receptors (TRs) to regulate gene transcription. Factors like estrogen status, inflammation, or genetic variations in binding proteins directly alter the percentage of free hormone available. Therefore, optimizing the balance between bound and unbound T is the mechanism of action for functional thyroid health.

Free T Bioavailability ∞ Area

Free T Bioavailability

Meaning

Free T Bioavailability refers to the concentration of unbound, metabolically active thyroxine (T4) and triiodothyronine (T3) circulating in the serum, available to diffuse into target tissues. This fraction, rather than total hormone, dictates true thyroid hormone effect on cellular metabolism. Clinical assessment often focuses on Free T3 as the most potent signaling molecule. Ensuring adequate bioavailability is paramount for maintaining euthyroid status across all systems. Low free T often presents clinically despite normal total T levels.