During pregnancy, every part of your body is affected. Your blood flow increases. Your lungs work harder. Even your hair grows faster. So it should come as no surprise that the thyroid gland is affected, too.

In pregnancy, two hormones in particular can affect the thyroid: human chorionic gonadotropin (HCG) and estrogen. HCG is a hormone produced early on in pregnancy by the placenta. In fact, most pregnancy tests work by detecting the presence of HCG. During pregnancy, HCG behaves similarly to TSH and can sometimes cause the thyroid gland to become slightly larger. Usually, this enlargement isn't significant and is not even noticeable except on ultrasound. The HCG can also sometimes cause TSH levels to dip in the first trimester, so at that time, your thyroid tests will show a slightly low TSH and slightly elevated T4. Your dose of thyroid hormone, however, is unlikely to change.

Estrogen goes up in the second and third trimesters and causes an increase in TBG, the protein that binds thyroid hormone. When TBG goes up, the amount of total T4 and T3 goes up. The increase in total T4 and T3 usually does not affect levels of free T4 and T3, which are the hormones actually available to your body cells.

For patients without hypothyroidism, the body compensates and no treatment is needed. The mother's blood gets diluted as it goes to the placenta and the baby. However, extra TBG binds to levothyroxine in patients who take thyroid hormone replacement. That's why women on thyroid hormone replacement typically need higher doses during the second half of pregnancy.

For a pregnancy to occur and to be healthy, it's important to maintain normal levels of TSH, free T4, and free T3. For healthy women, this comes naturally, without extra effort. But if you have a thyroid disorder, you may need extra medical intervention to achieve and maintain this healthy state of affairs.