Thyroid disorders may be present before women become pregnant, or they may develop during pregnancy. Being pregnant does not change the symptoms of thyroid disorders. How the fetus is affected depends on which thyroid disorder is present and which drugs are used for treatment. But generally, the following are risks:
The most common causes of hypothyroidism in pregnant women are Hashimoto's thyroiditis and treatment of Graves' disease.
If women have or have had a thyroid disorder, they and the baby are closely monitored during and after pregnancy. Doctors regularly check them for changes in symptoms and do blood tests to measure thyroid hormone levels.
This chronic inflammation of the thyroid gland is caused by an autoimmune reaction—when the immune system malfunctions and attacks its own tissues. Because the immune system is suppressed during pregnancy, this disorder may become less evident. However, pregnant women sometimes develop hypothyroidism or hyperthyroidism that requires treatment.
This sudden inflammation of the thyroid gland is common during pregnancy. The thyroid gland may enlarge, forming a goiter, and become tender. The goiter usually develops during or after a respiratory infection. Hyperthyroidism may develop and cause symptoms, but it is temporary. Subacute thyroiditis usually requires no treatment.
In the first few weeks after delivery, the thyroid gland may become suddenly inflamed, making the thyroid temporarily overactive. This disorder may be caused by an autoimmune reaction. The disorder may persist, recur periodically, or steadily worsen.
Abnormal antibodies stimulate the thyroid gland to produce excess thyroid hormone. These antibodies can cross the placenta and stimulate the thyroid gland in the fetus. As a result, the fetus may have a rapid heart rate and may not grow as much as expected. The fetus's thyroid gland may enlarge, forming a goiter. Very rarely, a goiter is so large that it interferes with delivery through the vagina.
Usually during pregnancy, Graves' disease is treated with the lowest possible dose of propylthiouracil, taken by mouth. Physical examinations and measurements of thyroid hormone levels are done regularly because propylthiouracil crosses the placenta. The drug may slow the activity of the thyroid gland and prevent the fetus from producing enough thyroid hormone. It may also cause a goiter to form in the fetus. Synthetic thyroid hormones, usually also used to treat this disorder, are not used with propylthiouracil during pregnancy. These hormones may cover up problems that occur when doses of propylthiouracil are too high and may cause hypothyroidism in the fetus. Methimazole may be used instead of propylthiouracil.
Often, Graves' disease becomes less severe during the 3rd trimester, so the drug dose can be reduced or the drug can be stopped.
Radioactive iodine, used to diagnose or treat Graves' disease, is not used during pregnancy because it can damage the fetus's thyroid gland.
If a thyroid storm (sudden, extreme overactivity of the thyroid gland) occurs or symptoms become severe, women may be given beta-blockers (used to treat high blood pressure). If necessary, the thyroid gland of pregnant women may be removed during the 2nd trimester. Women thus treated must begin taking synthetic thyroid hormones 24 hours after surgery. For these women, taking these hormones causes no problems for the fetus.
Last full review/revision December 2008 by Sean C. Blackwell, MD