The Thyroxine-Binding Globulin (TBG) blood test measure the patient’s TBG levels. Found in the blood, TBG is a protein necessary for transporting the thyroid hormone.
Alterations of TBG can typically be resolved by the thyroid profile test, but occasionally TBG must be directly measured. Testing Thyroxine-binding Globulin helps distinguish between high T4 levels due to hyperthyroidism, or due to increased binding by TBG in euthyroid (having normal thyroid gland function) individuals with normal levels of free hormones. Additionally, a TBG blood test helps document cases of hereditary deficiency or increase of TBG and work-up of thyroid disease.
Measurements of TBG are only occasionally needed in patients with low T4, high T3 or the reverse, who clinically seem eumetabolic and have normal FTI (Free Thyroxine Index or T7). These patients may have hereditary anomalies of TBG.
TBG levels increase due to pregnancy, estrogens, tamoxifen, perphenazine and in some cases of liver disease, including hepatitis. Decreased TBG is found in some instances of nephrosis, chronic liver disease and systemic disease, and with large amounts of glucocorticoids, acromegaly and androgens/anabolic steroids.