Thyroid Health and Pregnancy

Introduction

Thyroid health during pregnancy is very important for both the mother and the fetus (unborn child). During pregnancy the mother’s thyroid hormone crosses the placenta [pluh-SEN-tuh] and is needed for the growth of the fetus. When the mother’s thyroid produces either too little or too much thyroid hormone, serious side effects can occur. So, it is extremely important that during pregnancy the mother’s thyroid is working normally.

During the last 20 years there has been much research on the effect of thyroid disease on the mother and developing child. The research has shown that women may develop thyroid problems for the first time during pregnancy. To promote thyroid health during pregnancy, the American Thyroid Association published guidelines in October 2011 on all aspects of thyroid health during pregnancy (http://thyroidguidelines.net/pregnancy). The guidelines have been carefully reviewed and endorsed by the American Association of Clinical Endocrinologists [en-doh-cri-NA-lo-jists].

What changes happen with the mother’s thyroid gland during pregnancy? Pregnancy can be viewed as a “thyroid stress test.” During pregnancy the thyroid gland must make 50% more thyroid hormone. To do this, the mother also has to increase her intake of iodine (which is needed by the thyroid gland to make thyroid hormone) by 50%. So, the guidelines recommend that all pregnant and breastfeeding women take a daily prenatal vitamin that contains 150 mcg of iodine. This is very important because recent studies suggest that many women of childbearing age in the United States have low iodine levels.

Hypothyroidism (an underactive thyroid )

There are 2 types of hypothyroidism [hie-po-THIGH-roid-is-m]: minimal to mild, which is called subclinical hypothyroidism, and more severe, which is called overt hypothyroidism. Overthypothyroidism can lead to a miscarriage, preterm delivery, decreased IQ in the unborn child, and gestational [jeh-STAY-shun-ull] hypertension (high blood pressure during pregnancy). Subclinical hypothyroidism has also been associated with miscarriage, preterm delivery, and decreased IQ.

Women with hypothyroidism and on thyroid hormone replacement usually need to increase their dose during pregnancy. This is not surprising as their thyroid gland is unable to produce the extra thyroid hormone required during pregnancy. Before getting pregnant, women on levothyroxine [le-vo-thigh-ROX-een] (the typical thyroid-replacement drug) should have their hormone levels checked so their dose can be changed if needed. Pregnant women on this drug need to be checked often during pregnancy. The guidelines recommend how and when to adjust the dose of thyroid hormone, and how often to test the thyroid level during pregnancy.

In addition to women known to be hypothyroid prior to pregnancy, another 2.5% of all pregnant women are hypothyroid. Most of these women have subclinical hypothyroidism and do not have any symptoms. These women are unable to produce the extra 50% of hormone needed during pregnancy because their thyroid has already been partially damaged—usually by the body’s immune system attacking the thyroid gland and damaging it (www.ThyroidAwareness.com/thyroid-conditions). While the ATA guidelines do not recommend treatment in all women with subclinical hypothyroidism, treatment is recommended in women with subclinical hypothyroidism who have thyroid antibodies. This is checked with an easy blood test. A positive test means that a body’s immune system is attacking the thyroid gland. Women who get the test and get the right treatment have less chance for miscarriage and preterm delivery.

Whether or not to screen all women for thyroid disease during pregnancy has been, and remains, hotly debated. There is not much proof that treating pregnant women with subclinical hypothyroidism decreases complications in the mother and fetus. Therefore, the guidelines do not recommend universal screening. However, the guidelines do recommend that screening be done in all women in the following high-risk groups for thyroid disease:

High Risk Groups (not all high risk groups noted)

  • Women with a history of thyroid disease or thyroid surgery
  • Women over 30 years of age
  • Women with symptoms of thyroid disease or with a large thyroid known as a goiter
  • Women with type 1 diabetes or any other autoimmune disorder
  • Women with a history of miscarriage or preterm delivery
  • Women with a family history of thyroid disease
  • Women with infertility

Hyperthyroidism (an overactive thyroid )

Hyperthyroidism [hie-per-THIGH-roid-is-m] is also divided into overt and subclinical hyperthyroidism. It is much less common then hypothyroidism, and less than 1.0% of all pregnant women have it. Overthyperthyroidism causes miscarriage, gestational hypertension, eclampsia [eh-KLAMP-see-uh] (also known as toxemia and causes high blood pressure) and preterm delivery. Subclinical hyperthyroidism does not cause any bad outcomes in either the mother or unborn child.

The most common cause of hyperthyroidism during pregnancy is Graves’ disease, which is also an autoimmune disease. Graves’ disease is caused by an antibody in the blood that makes the thyroid gland release too much thyroid hormone. Women with Graves’ disease should see their doctor before getting pregnant to ensure that they are on the appropriate dose of anti-thyroid drugs. Even women successfully treated for Graves’ disease in the past require special monitoring during pregnancy. The ATA guideline offers specific recommendations for treating women with Graves’ disease during pregnancy, including how often to monitor the patient, the optimal drugs to use during the pregnancy, and the safety of breast feeding while on antithyroid medication.

Postpartum Thyroiditis

Postpartum thyroiditis (PPT), another autoimmune condition, is a thyroid disease in the postpartum period in women who typically do not have a history of having a thyroid disorder. Eight percent of all women (or approximately one out of every 12 women) will develop PPT. Women with postpartum [post-PAR-tum] thyroiditis [thigh-roid-EYE-tis] may be diagnosed any time during the first year following childbirth. If the diagnosis is made in the early stages of the condition, an overactive state (too much thyroid hormone, called hyperthyroidism) is more likely. During later stages of the condition, the thyroid runs out of thyroid hormone and becomes an underactive thyroid (too little thyroid hormone, called hypothyroidism). Most women will recover fully and have normal thyroid function at the end of the first year after childbirth. The ATA guidelines provide recommendations for the treatment of both the hyperthyroid and hypothyroid phases. The guidelines also recommend yearly monitoring in women who had an episode of PPT because they are at higher risk for getting permanent hypothyroidism.

Conclusion

Thyroid health during pregnancy is important for the mother and developing baby. All pregnant women should take prenatal vitamins with iodine. Women with pre-existing thyroid disease need special monitoring and treatment during pregnancy. Because both hypothyroidism and hyperthyroidism cause serious side effects, first trimester screening of women at high risk for thyroid disease is recommended.

Dr. Stagnaro-Green graduated from Mount Sinai School of Medicine. Following a residency in Internal Medicine at New Jersey Medical School he completed a fellowship in Endocrinology and Metabolism at the Mount Sinai School of Medicine. In 2005, he received a Masters of Health Professions Education at the University of Illinois-Chicago. Dr. Stagnaro-Green is an internationally known researcher in the field of thyroid disease and pregnancy. His endocrine research focuses on thyroid antibodies and miscarriage, the relationship between thyroid dysfunction and preterm delivery, and postpartum thyroiditis. Presently he is senior associate dean for education at the George Washington University School of Medicine and Health Sciences where he is Professor of Medicine and Professor of Obstetrics and Gynecology.