What effect does increased estrogen during pregnancy have on TBG levels?

During pregnancy, increased estrogen levels lead to a physiological response that includes the elevation of thyroxine-binding globulin (TBG) levels. Estrogen stimulates the liver to produce more TBG, resulting in higher circulating levels of this protein.

The increase in TBG is significant because it affects the total levels of thyroid hormones in the bloodstream. Although the free (biologically active) thyroid hormone levels remain tightly regulated, the total T4 and T3 levels measured in the blood can rise due to the increased binding capacity from elevated TBG. This increase is crucial for ensuring that adequate thyroid hormones are available for both the developing fetus and the mother.

In summary, the correct response signifies that estrogen, through its action on TBG production, plays a vital role in maintaining adequate thyroid hormone levels during pregnancy, which is essential for both maternal and fetal health.