In pregnant patients, which thyroid hormone levels typically change?

In pregnancy, the thyroid hormone dynamics are influenced by several physiological changes, particularly concerning thyroxine-binding globulin (TBG) levels. During pregnancy, estrogen levels rise, which stimulates the liver to produce more TBG. This increase in TBG leads to higher total levels of thyroid hormones (T3 and T4), as more of these hormones become bound to TBG. However, it's important to note that while total T3 and T4 can be elevated due to this binding, the free forms of T3 and T4—those that are biologically active and unbound—might not increase significantly.

The increase in TBG is significant for understanding thyroid function during pregnancy because it affects the interpretation of thyroid hormone levels. Free T3 and free T4 levels often remain within the normal range in healthy pregnant women, even as total hormone levels rise. Thus, the elevation of TBG is a crucial adjustment that helps maintain normal physiological thyroid function as the metabolic needs of both the mother and the developing fetus change.

In this context, mentioning that normal TSH levels may also be observed during early pregnancy, despite changes in TBG, emphasizes how the body adapts to maintain homeostasis in thyroid hormone levels, especially relevant when assessing thyroid function in