Thyroxine Binding Globulin (TBG) is a protein produced in the liver that binds to thyroid hormones T3 and T4, which circulate in the bloodstream.
TBG levels can affect the total levels of T3 and T4 in the blood, as the majority of these hormones are bound to TBG. However, TBG levels do not affect the levels of free T3 and T4, which are the active forms of thyroid hormones that are available for use by the body’s cells.
TBG levels can be affected by certain medical conditions and medications, such as liver disease, estrogen therapy, and pregnancy. Therefore, measuring TBG levels may be useful in diagnosing and monitoring these conditions, as well as in interpreting thyroid function tests.
However, TBG testing is not routinely performed as part of thyroid function testing, as it is considered a specialized test and may not provide additional information beyond that obtained by other thyroid function tests, such as TSH, Free T3, and Free T4. Your doctor may recommend TBG testing if there is suspicion of a specific medical condition or medication affecting TBG levels.