In the circulation, thyroid hormones such as thyroxine (T4) and triiodothyronine (T3) are bound to a protein called thyroid binding globulin (TBG), which is generated in the liver. TBG’s principal role is to carry thyroid hormones to their target tissues. The measurement of TBG count can be helpful in evaluating thyroid function.

Low TBG levels may indicate the following:

• Liver disease: Liver dysfunction or injury can result in decreased TBG synthesis, which in turn reduces TBG levels in the blood.
• High quantities of androgen hormones, as in androgen treatment, can inhibit TBG synthesis and thereby reduce TBG levels.

An elevated TBG level could indicate:

• Increased TBG levels can result from inherited genetic variants that induce a disorder called familial dysalbuminemic hyperthyroxinemia (FDH).
• Hormonal shifts occur during pregnancy, causing TBG levels to increase.
• Contraceptives and estrogen therapy: Estrogen can stimulate TBG synthesis, leading to elevated blood levels of this hormone.