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Thyroxine-binding globulin (TBG) is a globulin protein encoded by the SERPINA7 gene in humans. TBG binds thyroid hormones in circulation . It is one of three transport proteins (along with transthyretin and serum albumin ) responsible for carrying the thyroid hormones thyroxine (T 4 ) and triiodothyronine (T 3 ) in the bloodstream.
Total thyroxine is rarely measured, having been largely superseded by free thyroxine tests. Total thyroxine (Total T 4) is generally elevated in hyperthyroidism and decreased in hypothyroidism. [2] It is usually slightly elevated in pregnancy secondary to increased levels of thyroid binding globulin (TBG). [2]
Thyroid hormone binding ratio (THBR) is a thyroid function test that measures the "uptake" of T3 or T4 tracer by thyroid-binding globulin (TBG) in a given serum sample. This provides an indirect and reciprocal estimate of the available binding sites on TBG within the sample. The results are then reported as a ratio to normal serum.
Thyroglobulin (Tg) is a 660 kDa, dimeric glycoprotein produced by the follicular cells of the thyroid and used entirely within the thyroid gland. Tg is secreted and accumulated at hundreds of grams per litre in the extracellular compartment of the thyroid follicles, accounting for approximately half of the protein content of the thyroid gland. [5]
T 4, Thyroxine (3,5,3′,5′-tetraiodothyronine), is produced by follicular cells of the thyroid gland. It is produced from the precursor thyroglobulin (this is not the same as thyroxine-binding globulin (TBG)), which is cleaved by enzymes to produce active T 4. [34] The steps in this process are as follows: [31]
It's bound to proteins like albumin and sex hormone binding globulin , which helps transport testosterone but prevents the body from using it right away. ... or SHBG is too high, patients can ...
After secretion, only a very small proportion of the thyroid hormones travel freely in the blood. Most are bound to thyroxine-binding globulin (about 70%), transthyretin (10%), and albumin (15%). [30] Only the 0.03% of T 4 and 0.3% of T 3 traveling freely have hormonal activity. [31]
During pregnancy, there is also an increase in estrogen which causes the mother to produce more thyroxine binding globulin, which is what carries most of the thyroid hormone in the blood. [24] These normal hormonal changes often make pregnancy look like a hyperthyroid state but may be within the normal range for pregnancy, so it necessary to ...