What effect does oral estrogens have on thyroid binding globulin?

Oral estrogens are known to influence the levels of various proteins in the body, one of which is thyroid-binding globulin (TBG). When estrogens are introduced into the system, they enhance the production of TBG in the liver. This results in an increase in the levels of TBG in the bloodstream.

The primary role of TBG is to transport thyroid hormones—specifically thyroxine (T4) and triiodothyronine (T3)—to tissues throughout the body. An increase in TBG means there are more binding sites available for thyroid hormones, which can lead to higher total levels of thyroid hormones in the bloodstream. However, free (active) hormone levels may remain stable, as the body typically maintains homeostasis.

Understanding the effect of estrogen on TBG is essential for recognizing how hormonal changes can affect thyroid hormone levels and their bioavailability, particularly in women experiencing menopause or undergoing hormone replacement therapy.