Hypothyroidism and Pregnancy

By Alex S tagnaro-Green, MD, MHPE

Many women who become pregnant are on thyroid hormone replacement. The most common reason that a woman of childbearing age is on thyroid hormone is due to a disease called Hashimoto’s [ha-SHI-mo-toz] thyroiditis [theye-royd-EYET-uhss], a condition where the body’s own immune system becomes confused and destroys the thyroid. Other reasons why women are on thyroid hormone include the need to replace the hormone following surgery (for thyroid cancer or thyroid nodules) or following destruction of the gland by radioiodine treatment (for an overactive thyroid condition called Graves' disease).

No matter the reason a pregnant woman is on thyroid hormone, it is critical that she be on the correct dose of medicine both prior to and throughout her pregnancy. Studies over the last 20 years have shown that is important, for the health of the fetus and the mother, that the pregnant woman have normal thyroid function throughout the entire pregnancy. According to the studies, pregnant women who do not receive enough thyroid hormone are more likely to have a number of complications including maternal hypertension, miscarriage, preterm delivery and decreased IQ in their child.

Maintaining a normal state of thyroid function during pregnancy requires careful attention by both the physician and mother-to-be, as dramatic changes occur in the thyroid during this time. During pregnancy, in a woman who has no thyroid disease, the thyroid gland needs to dramatically increase the amount of thyroid hormone that is made. Specifically, the thyroid gland has to produce 50 percent more thyroid hormone. There are many reasons why this is the case, including the fact that the mother is producing thyroid hormone both for herself and for the developing baby. Women who have a thyroid gland that functions normally have no difficulty in producing the extra hormone needed for the unborn child, although it is important that all pregnant women take a prenatal vitamin with a sufficient amount of iodine, as iodine is necessary for the thyroid gland to produce thyroid hormone. Not all prenatal vitamins contain iodine, so be sure to check labels properly.

However, the situation is completely different in women who are on thyroid hormone prior to pregnancy. In these individuals, since the thyroid gland is no longer functioning (due to Hashimoto’s thyroiditis, surgery or radioactive iodine treatment) the thyroid cannot respond to pregnancy by producing more hormone. The majority of these women will develop an underactive thyroid– called hypothyroidism [hi-po-thi-royd-ism]–once they become pregnant which, as discussed, may have a negative impact on the both the developing baby and the mother. In fact, studies have shown that approximately two-thirds of all women on thyroid hormone replacement will develop some degree of hypothyroidism during pregnancy if appropriate steps are not taken.

So, what needs to be done? The most important step is to talk with your physician about your thyroid condition prior to becoming pregnant. Frequently, your physician will increase the dose of thyroid hormone even prior to you attempting to conceive. Secondly, you should contact your physician as soon as you confirm that you are pregnant. At that point, your doctor will either have you increase the dose of your thyroid medicine or have you come in immediately for a blood test. Frequent monitoring of your thyroid status, especially during the first half of pregnancy, will also be needed to ensure that your thyroid function remains in the normal range of pregnancy.

With appropriate planning, working closely with your physician prior to pregnancy and ongoing communication between you and your physician throughout the pregnancy, you can optimize your thyroid health and that of your unborn child.

Dr. Stagnaro-Green is Professor of Medicine and Professor of Obstetrics-Gynecology at the George Washington University School of Medicine and Health Sciences. He received his M.D. from the Mount Sinai School of Medicine in New York City and in 2005 received a Master of Health Professions Education degree from the University of Illinois-Chicago. Dr. Stagnaro-Green has been involved in research in thyroid and pregnancy for over 20 years and chaired an international committee for the American Thyroid Association which published comprehensive guidelines on thyroid and pregnancy in 2011.