Thyroxine-binding globulin (TBG) is a protein that is produced in the liver and plays a key role in the transport of thyroid hormones, specifically thyroxine (T4), in the bloodstream. TBG is one of three major thyroid hormone-binding proteins in the blood, the other two being transthyretin (prealbumin) and albumin.
The majority of T4 in the blood is bound to TBG, with only a small fraction of free, unbound T4 being available to enter cells and exert its biological effects. Changes in the level of TBG in the blood can therefore affect the overall availability and activity of thyroid hormones in the body.
Certain factors can increase or decrease TBG levels in the blood. For example, estrogen can increase TBG production, leading to higher levels of bound T4 in the bloodstream. In contrast, certain medications, such as glucocorticoids, can decrease TBG production, leading to lower levels of bound T4.
Measuring TBG levels in the blood can be useful in certain clinical settings, such as in the evaluation of thyroid function in pregnant women or in individuals with suspected thyroid hormone-binding protein abnormalities. However, it is important to note that TBG levels can be affected by a variety of factors, including medications and other health conditions, and should be interpreted in the context of a person’s overall health and medical history. A healthcare provider can help to interpret TBG results and determine if further testing or treatment is necessary.