Thyroid Hormone Transport and Binding Proteins

Once secreted, T4 and T3 are highly hydrophobic and circulate almost entirely bound to plasma carrier proteins, with only a small free fraction available to tissues. Delivery into cells then depends on specific membrane transporters. This topic covers both the plasma binding proteins that buffer and distribute hormone and the transmembrane carriers that move it into target cells.

Definition

Thyroid hormone transport refers to the carriage of T4 and T3 in plasma by binding proteins and their subsequent transmembrane uptake into cells by specific hormone transporters.

Scope

This topic covers the major serum thyroid hormone binding proteins (thyroxine-binding globulin, transthyretin, and albumin), the free-hormone hypothesis that links bound and free pools, and the membrane transporters such as MCT8, MCT10, and the organic anion transporting polypeptides that mediate cellular uptake. It is a physiological reference and does not address the clinical interpretation of binding-protein abnormalities.

Core questions

• Which plasma proteins carry T4 and T3, and in what proportions?
• What is the relationship between bound hormone and the biologically available free fraction?
• How does hormone cross the plasma membrane into target cells?
• Why does cellular entry require dedicated transporters rather than simple diffusion?

Key concepts

• Thyroxine-binding globulin (TBG)
• Transthyretin (TTR)
• Albumin as low-affinity carrier
• Free-hormone hypothesis
• Bound versus free hormone pools
• Monocarboxylate transporter 8 (MCT8)
• MCT10 and OATP transporters

Mechanisms

In plasma, the great majority of T4 and T3 is reversibly bound to thyroxine-binding globulin, with smaller contributions from transthyretin and albumin; these proteins act as a circulating reservoir that buffers free hormone concentrations and smooths delivery to tissues. Under the free-hormone hypothesis, it is the small unbound fraction that is metabolically active and available for cellular uptake, so changes in binding-protein concentration shift total hormone while tending to leave free hormone regulated. Cellular entry is not purely passive: specific transmembrane carriers, including monocarboxylate transporter 8, MCT10, and organic anion transporting polypeptides, move hormone across the plasma membrane so that it can reach intracellular deiodinases and nuclear receptors.

Clinical relevance

The distinction between bound and free hormone underlies why free T4 and free T3 measurements can differ from total hormone when binding proteins change, and why cellular transporters are required for hormone to reach its targets. This entry is descriptive physiology and is not guidance for evaluating or managing binding-protein or transporter abnormalities.

History

Early biochemical work characterized the serum thyroid hormone binding proteins and led to the free-hormone hypothesis, which reframed hormone action around the unbound fraction rather than total concentration. The later identification of specific membrane transporters, especially MCT8, showed that cellular uptake is an active, carrier-mediated step and not the simple diffusion long assumed for lipophilic hormones.

Key figures

• Luigi Bartalena
• Theo J. Visser
• Robin P. Peeters
• Paul M. Yen

Seminal works

• bartalena-1990
• vanderdeure-2010

Frequently asked questions

What is the free-hormone hypothesis?

It holds that only the small fraction of thyroid hormone not bound to plasma proteins is metabolically active and available to enter cells, so the free concentration, rather than the total, determines hormone action.

Why do cells need thyroid hormone transporters?

Although thyroid hormones are lipophilic, efficient and regulated entry into cells depends on specific membrane transporters such as MCT8, which carry hormone to intracellular deiodinases and nuclear receptors.