TBG - TBG values

Generality

Thyroxine-binding globulin (TBG) is a protein capable of binding and transporting thyroid hormones in the blood.

TBG exhibits a "high affinity" for the hormone thyroxine (T4); the interaction with triiodothyronine (T3) is less stable.

In any case, the binding of T3 and T4 with TBG is reversible, and aimed at maintaining an adequate concentration of the aforementioned hormones. in free form (FT3 and FT4). It should be remembered, in fact, that in the blood most of the thyroid hormones are bound, while only a small percentage is free and available for the tissues.
An increase in TBG can result in an increase in total T4 and T3 without an increase in hormone activity in the body. If a further thyroid hormone test is indicative of symptom-free hypo- or hyperthyroidism, thyroxine-binding globulin levels become higher. clinically relevant.
Abnormalities of thyroid hormone-TBG interaction can result from:

• Hormone-protein binding defect; in this case, the control of thyroid hormone secretion is preserved and the pituitary-thyroid axis is normal.
• Primary alterations in the plasma concentration of thyroid hormones, as occurs, for example, in "hypothyroidism or thyrotoxicosis. In this case, the normal homeostatic balance of hormonal secretion is lost, both due to a defect in the control mechanism itself, and for the" inability to counterbalance the effects of the underlying disease.

What's this

TBG is the acronym for Thyroxine-binding globulin (thyroxine-binding globulin); it is a glycoprotein with a molecular weight of 60,000 Dalton, responsible for the transport of thyroid hormones, T3 and T4, in the blood.

TBG is synthesized by the liver and has a single binding site in its structure, for both T3 and T4.

Despite the reduced plasma concentrations, TBG binds to itself almost all thyroid hormones (70-80%), which to a lesser extent are associated with two other proteins, also synthesized by the liver: albumin and transthyretin (TTR or pre-albumin fraction binding T4 - TBPA).

To an even smaller extent, thyroid hormones are found free in the blood: only about 0.02-0.04% of T4 and about 0.3-0.4% of T3.

The need to convey thyroid hormones by means of special transport proteins arises from their lipophilic nature, which makes them insoluble in water-based liquids, such as blood. However, in order to acquire biological activity and regulate metabolism in target cells, thyroid hormones must necessarily detach from these carrier proteins; this is why for some years it has been preferred to measure the plasma levels of the free fraction (free T4 and T3, often indicated in the certificate of analysis as FT3 and FT4), rather than the absolute ones (total T3 and T4).
Let's try to clarify the concept better.