In pregnant women, what contributes to increased transport proteins affecting thyroid hormone levels?

During pregnancy, elevated estrogen levels play a significant role in the increase of transport proteins, specifically thyroxine-binding globulin (TBG), which impacts thyroid hormone levels in the body. Estrogen stimulates the liver to produce more TBG, leading to a higher concentration of this protein in the blood. As TBG binds to thyroid hormones (such as thyroxine, T4), this results in an overall increase in the total serum levels of these hormones, even though the levels of free, biologically active thyroid hormones may remain stable or even decrease temporarily.

The increase in TBG due to heightened estrogen levels means that the body may have an increased storage capacity for thyroid hormones, which can affect the interpretation of thyroid function tests. It is important for healthcare providers to consider this physiological change when evaluating thyroid function during pregnancy. Therefore, recognizing the impact of elevated estrogen on transport proteins is crucial for understanding how thyroid hormone levels are regulated in pregnant women.