In a pregnant woman, which hormone primarily influences the changes observed in thyroid function tests?

During pregnancy, the hormone that primarily influences the changes observed in thyroid function tests is estrogen. Estrogen levels rise significantly during pregnancy, which has several effects on the thyroid gland and the thyroid hormones.

One of the key roles of estrogen is its effect on thyroid-binding proteins, specifically thyroxine-binding globulin (TBG). Estrogen increases the synthesis of TBG in the liver, which leads to an increase in the total levels of circulating thyroid hormones (thyroxine [T4] and triiodothyronine [T3]), even while the free or active forms of these hormones may remain within normal ranges. This results in alterations in thyroid function tests, which can show elevated total T4 and T3 levels, while free hormone levels might not be as elevated, reflecting the increased binding capacity due to the higher levels of TBG.

The physiology of thyroid hormone regulation during pregnancy is important because it helps to ensure adequate thyroid hormone supply for both the mother and the developing fetus. Adequate levels of thyroid hormone are crucial for fetal brain development, especially during the first trimester when the fetus is entirely dependent on maternal thyroid hormones.

Understanding the impact of estrogen on thyroid function is essential for interpreting thyroid function tests in pregnant women accurately, given the