Thyroxine-binding globulin (TBG) is a protein that is produced by the liver and binds to thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), in the bloodstream. TBG is one of three major thyroid hormone-binding proteins, the other two being transthyretin (TTR) and albumin.
The function of TBG is to transport thyroid hormones throughout the body, helping to regulate metabolism and other physiological processes. Because TBG has a high affinity for thyroid hormones, it helps to keep the concentration of free (unbound) thyroid hormones in the blood at a stable level.
Changes in TBG levels can affect the concentration of free thyroid hormones in the blood and may lead to conditions such as hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone). For example, increased levels of TBG can cause total T4 and T3 levels to appear high, even though the levels of free T4 and T3 are normal. Conversely, decreased levels of TBG can cause total T4 and T3 levels to appear low, even though the levels of free T4 and T3 are normal.
Measuring TBG levels may be useful in diagnosing certain thyroid disorders, and can help healthcare providers to better understand a patient’s thyroid hormone status.