Thyroxine-binding globulin Totally Explained

Everything about Thyroxine-binding Globulin totally explained

| Name = Serpin peptidase inhibitor, clade A (alpha-1 antiproteinase, antitrypsin), member 7 | HGNCid = 11583 | Symbol = SERPINA7 | AltSymbols =; TBG | OMIM = 314200 | ECnumber = | Homologene = 20113 | MGIid = 3041197 | GeneAtlas_image1 = PBB_GE_SERPINA7_206386_at_tn.png | Function = | Component = | Process = | Orthologs = }} Thyroxine-binding globulin (TBG) binds Thyroid hormone in circulation. It is one of three proteins (along with transthyretin and albumin) responsible for carrying the thyroid hormones thyroxine (T4) and 3,5,3’-triiodothyronine (T3) in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3, but is present in the lowest concentration. Despite its low concentration, TBG carries the majority of T4 in serum. Due to the very low serum concentration of T4 & T3, TBG is rarely more than 25% saturated with its ligand. Unlike transthyretin and albumin, TBG has a single binding site for T4/T3. TBG is synthesized primarily in the liver as a 54 kDa protein. Genomically, TBG is a serpin, although it has no inhibitory function like many other members of this class of proteins.

Role in Diagnosis

TBG tests are sometimes used in finding the reason for elevated or diminished levels of thyroid hormone. This is done by measuring resin binding to labeled thyroid hormone, which only happens when the labeled thyroid hormone is free.
The patient's serum is mixed with the labeled thyroid hormone; then, the resin is added to the whole mixture, to measure the amount of free labeled thyroid hormone. So, for instance, if the patient is truly hypothyroid, and TBG levels are normal, then there are many sites open for binding on the TBG, since the total thyroid hormone level is low. Therefore, when the labeled hormone is added, it'll mostly bind to the TBG, leaving little of it left for binding to the resin. Conversely, if the patient is truly hyperthyroid, and TBG levels are normal, the patient's endogenous hormone will saturate the TBG binding sites more, leaving less room for the labeled hormone, which allows greater binding to the resin.
In the situations described above, TBG testing isn't very useful. However, total thyroid hormone levels point to hypothyroidism or hyperthyroidism, without the expected symptoms, the utility of TBG testing becomes more evident, since TBG production can be modified by other factors such as estrogen, corticosteroids, or liver failure. If, for example, the TBG level is high, which can occur when estrogen levels are high, the TBG will bind more thyroid hormone, decreasing the free hormone available in the blood, which leads to stimulation of TSH, and the production of more thyroid hormone. In this case, the total thyroid hormone level will be high. However, when labeled hormone is added, since TBG is so high, it'll bind to the TBG, leaving little free labeled hormone for uptake into the resin. Conversely, in the presence of corticosteroids, which lower TBG levels, the total thyroid hormone (bound and free) in the blood will be low. However, when the labeled hormone is added, since so little TBG is available in the blood, only a small portion of it'll bind, leaving plenty available for uptake by the resin.

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