Also known as: TBG, Thyroxine (T4), and T4 − TBG Index.

Special Note: Test Results take at least 5 days

Thyroid Binding Globulin (TBG) - Distinguishes between high T4 levels due to hyperthyroidism and due to increased binding by TBG in euthyroid individuals who have normal levels of free hormones; documents cases of hereditary deficiency or increase of TBG; work-up of thyroid disease. In those with low T4, high T3 (uptake) or the reverse, who clinically seem eumetabolic and have normal FTI, measurement of TBG is only occasionally needed. Some patients may have hereditary anomalies of TBG. TBG is increased by estrogens, tamoxifen, perphenazine, pregnancy, and in some cases of liver disease, including hepatitis. Decreased TBG is found with certain instances of chronic liver disease, nephrosis and systemic disease and with large amounts of glucocorticoids, androgens/anabolic steroids, and acromegaly. Although alterations of TBG are usually resolved by the thyroid profile, TBG must be directly measured occasionally.