Which condition is known to increase total T4 by raising TBG levels?

The correct answer is that pregnancy or estrogens are known to increase total T4 by raising thyroxine-binding globulin (TBG) levels. During pregnancy, the body undergoes significant hormonal changes, leading to an increase in TBG production. Estrogens are also known to stimulate the liver to produce more TBG, which binds thyroid hormones in the bloodstream. This increase in TBG results in a higher total T4 level, even if the free (active) T4 level remains unchanged.

This is an important aspect of thyroid function during pregnancy and hormone therapy because it helps ensure that there are adequate levels of thyroid hormones available for both the mother and the developing fetus. Monitoring total T4 in these scenarios is essential for appropriate management and assessment of thyroid function.