Blood & Laboratory Values

Thyroxine Binding Globulin – Function & Diseases

Thyroxine Binding Globulin

The thyroxine-binding globulin is a protein that binds the thyroid hormones triiodothyronine (T3) and L-thyroxine (T4) in the body. These play an important role in the energy metabolism in mammals.

What is Thyroxine Binding Globulin?

The thyroxin-binding globulin belongs to the group of globulins, storage and transport proteins in the blood plasma . The globulins are divided into four groups. This classification is based on the electrophoretic movement of these proteins in serum protein electrophoresis, a laboratory test of proteins in blood plasma.

The proteins are separated by electric charge. The α1-globulin group includes the thyroxine-binding globulin, but also transcortin, for steroid transport, prothrombin, which is involved in blood clotting, and transcobalamin, which binds vitamin B12. In addition, the Gc globulins, which bind vitamin D , bilirubin transporters and α1-antitrypsin belong to this group of globulins .

The α2-globulins consist of the α2 haptoglobin, the hemoglobin -binding globulins, the plasminogen, the α2-macroglobulin, the α2-antithrombin and the ceruloplasmin, which transports copper ions in the blood . The β-globulins are involved in the transport of lipids through the β-lipoproteins. In addition, transferrin for iron transport , fibrinogen and hemopexin for hemin binding and transport belong to this group. The γ-globulins consist of the immunoglobulins. These are also known as antibodies and serve to protect the body’s immune system .

Function, effect & tasks

The thyroxine-binding globulin binds the thyroid hormones triiodothyronine (T3) and L-thyroxine (T4). It serves to store and transport these hormones in the body to the various destinations where these hormones are needed. T3 and T4 are thyroid hormones based on the amino acid tyrosine , which are essential components of the energy metabolism.

It acts throughout the body and is involved in metabolism , protein biosynthesis , bone marrow growth, and neuron maturation . It also regulates the body’s sensitivity to catecholamines such as adrenaline. Furthermore, thyroxine is involved in fat, carbohydrate and vitamin metabolism. This shows how diverse the functions of thyroxine are and how important it is for it to reach its destination in the body efficiently through the transport protein. In the blood plasma, 99% of T3 and thyroxine-binding globulins are bound. L-thyroxine is also 99.9% bound to these. Both hormones are only freely available in small amounts, which is then referred to as free hormone.

However, thyroxine-binding globulin is not the only protein involved in T3 and T4 transport. This transport can also be taken over by transthyretin or serum albumin. However, thyroxine-binding globulin has a higher affinity for these thyroid hormones compared to the other two proteins. However, thyroxine-binding globulin occurs in human blood in smaller amounts than transthyretin or serum albumin .

It mainly binds and transports T4 in the blood plasma and is about 25% saturated with the thyroid hormones. This is due to the relatively low amount of thyroid hormones in the blood. These transport proteins protect the hydrophobic thyroid hormone from its watery environment in the blood. Thyroxine-binding globulin also belongs to the serpins, a family of serine protease inhibitors. Proteases are proteins that can break down other proteins into their individual parts.

Thyroid hormone is detached from globulin by being separated from it by active cleavage. This type of bond between the thyroid hormone and the globulin is called reversible because it can be reversed through active cleavage.

Formation, Occurrence, Properties & Optimal Values

Thyroxine-binding globulin is mainly produced in the liver . It is a 54 kDa protein that is initially synthesized as a polypeptide. This is followed by maturation and folding of the protein so that it becomes functional. The concentration of thyroxine-binding globulin in an adult is about 260 nmol/l. However, the concentration of this globulin depends on various factors.

The concentration can be increased by pregnancy, or by taking contraceptive medication or estrogen preparations. Various other drugs are known to increase levels of thyroxine-binding globulin, such as tamoxifen, which is used in the treatment of breast cancer opium-containing drugs. In addition, the concentration can also be increased by hepatitis or genetically.

A reduction in the concentration of thyroxine-binding globulin can be produced by other drugs such as glucocorticoids or androgens. In addition, the nephrotic syndrome or also genetic causes have a decreasing effect on the concentration of this globulin.

Diseases & Disorders

A relatively rare disease is chronic thyroxine-binding globulin deficiency. A distinction is made between the complete lack of thyroxine-binding globulin and the partial lack of this globulin. In complete deficiency there is a total loss of this globulin, in partial deficiency there is a reduction in the amount of globulin or a change in the structure of the protein.

This change in protein structure can have genetic causes. Due to these structural changes, the globulin is no longer able to bind and transport the thyroid hormone. Thyroid function is not affected by this condition. However, these diseases do not lead to any health problems, since this globulin is not the only protein that can bind and transport thyroid hormones.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.