The Impact of Thyroid Disease During Pregnancy

By Erik K. Alexander, MD and Elizabeth N. Pearce

Thyroid disease is relatively common in pregnant women. Hypothyroidism (too little thyroid hormone) occurs in at least 2 to 3 percent of pregnant women, while hyperthyroidism (too much thyroid hormone) occurs in up to 1 percent of all pregnancies.

Recently a national guideline was produced by the American Thyroid Association (ATA) that provided recommendations for the treatment of thyroid disease during pregnancy (www.thyroid.org/association-guidelines-management). Below we highlight some of the thyroid illnesses that can impact pregnancy and the recommendations for testing, treatment and prevention of these conditions.

Hypothyroidism and Pregnancy

Common symptoms of hypothyroidism include fatigue, weight gain, cold intolerance and constipation. Thyroid hormone from the mother is needed for growth and development of the unborn baby. During pregnancy, the amount of thyroid hormone required for both the mother and the developing baby increases by 40 percent, on average. So for women who have thyroid disease that has affected the ability to make enough thyroid hormone, or those who have had their thyroid destroyed by radioactive iodine or surgically removed, the gland no longer has the ability to increase thyroid hormone production as needed.

Therefore, it is very important that any woman with a history of hypothyroidism notify their physician or endocrinologist as soon as they know they are pregnant. Prompt testing of the woman’s thyroid hormone levels is extremely important, as it is very likely that the dosage of thyroid hormone medication will need to be adjusted. Additionally, testing blood levels at intervals throughout the pregnancy is crucial to make sure that normal thyroid hormone values are maintained throughout the pregnancy. Typically, testing thyroid function every four weeks through the first half of the pregnancy is recommended, with a general goal of maintaining a TSH (thyroid stimulating hormone) blood level less than 2.5mIU/L when on thyroid hormone replacement medication. Thyroidstimulating hormone is made by the pituitary gland in the brain and directs the thyroid to make and release thyroid hormone into the blood. TSH levels that are too high or too low can indicate your thyroid isn't working correctly.

A separate but important issue during pregnancy is the presence of an antibody called thyroid peroxidase antibody (TPO Ab). This antibody is very common in patients with Hashimoto’s disease, which is the most common cause of hypothyroidism (low thyroid function) and is present when the body’s immune system – normally designed to detect and attack things like bacteria that don’t belong in the body – mistakenly sees the thyroid as something “foreign” and makes antibodies that attack the thyroid tissue.

It is recommended that patients with hypothyroidism who are pregnant be tested not only for their thyroid function (serum TSH), but also for the presence of this TPO antibody. This is because the presence of the TPO antibody appears to increase the risk of miscarriage and other pregnancy complications. There is some clinical data that suggest that thyroid hormone replacement medication may reduce these pregnancy risks and that if the antibody is present, it is even more important to keep within the TSH range of less than 2.5mIU/L. This is a complex issue and should be thoroughly discussed with your physician.

The most important aspect of caring for pregnant women with hypothyroidism is to ensure they are aware of the importance of notifying their physician if they are planning pregnancy or suspect themselves newly pregnant. Frequent thyroid hormone checks throughout pregnancy, and appropriate adjustments to thyroid medication as is typically needed, can lead to a safe and successful pregnancy.

Hyperthyroidism and Pregnancy

Common symptoms of hyperthyroidism, such as fatigue, palpitations, anxiety and heat intolerance, can overlap with the symptoms of a normal pregnancy. In the first trimester, high levels of normal pregnancy hormones sometimes cause mildly high levels of thyroid hormone. This usually occurs in women with more severe nausea and vomiting (morning sickness). This form of pregnancyinduced hyperthyroidism usually improves after the first trimester as pregnancy hormone levels drop and does not need any treatment.

Graves’ disease is the most common type of hyperthyroidism in young women. In Graves’ disease, the immune system fails to recognize the thyroid gland as “self” and makes antibodies that attach to the thyroid gland and cause it to become overactive. Untreated hyperthyroidism from Graves’ disease can be dangerous both for a pregnant woman and her unborn child.

Both of the medicines used to treat Graves’ disease (methimazole and propylthiouracil, or PTU) can cause birth defects, although PTU is the safer of the two in the first trimester of pregnancy. Women with Graves’ disease who are planning a pregnancy should discuss the best treatment options with their doctor. One option is removing the thyroid with surgery or treating it with radioactive iodine treatment before conception so that Graves’ disease medicine is not needed in pregnancy. Another option is to continue medicine (usually PTU) during pregnancy, but to use the lowest possible dose. In some cases, patients with mild disease may be able to stop their medicine as soon as pregnancy is diagnosed. It is important to do this only after discussion with a physician and only when very close monitoring can be performed.

The Importance of Screening

Screening for the presence of thyroid disease in patients who are newly pregnant has been an active area of research and interest for many years, since thyroid disease is very common. Furthermore, as noted above, thyroid disease can adversely impact pregnancy as well as the health of the developing baby. So it might seem logical to recommend screening every pregnant woman for thyroid disease.

In the last few years, two large studies investigated the utility of such a screening recommendation. Surprisingly, the studies showed no benefit to early screening. Specifically, the ability to detect, intervene and normalize thyroid function in pregnant women with mild hypothyroidism did not demonstrate an improved outcome in the IQ of the child, which was the focus of earlier studies theorizing that babies born to mothers with undiagnosed or inadequately treated hypothyroidism are at risk for lower IQ scores and learning disabilities.

Because of this, and even though some in medical circles question whether thyroid hormone treatment was started early enough in pregnancy in these studies to have had an impact, current guideline recommendations do not support universal screening of all newly pregnant women for thyroid disease. However, the guidelines did support physicians asking all women who are planning pregnancy or are newly pregnant if they have a history of thyroid disease or risk factors for thyroid disease. If risk factors are present, then thyroid testing is recommended.

Risk factors include age greater than 35 years old, symptoms which could be related to thyroid dysfunction, symptoms which could be related to thyroid hormone excess/insufficiency, a family history of thyroid problems, and obesity among others. A full list of such risk factors can be found in the ATA guidelines document.

Iodine

Iodine is a nutrient that is needed to make thyroid hormone. Pregnant women need more iodine in their diets than nonpregnant women to maintain normal thyroid function. Even mild iodine deficiency in pregnant women has been linked to lower intelligence in their children. Globally, salt iodization (fortifying edible salt with iodine) has been the leading strategy for eliminating iodine deficiency. In the U.S., salt iodization has never been mandated by law, and most of the salt we eat is not iodized. While the population as a whole has been iodine sufficient since the 1940s, mild iodine deficiency has recently been identified in pregnant U.S. women.

Although blood and urine testing can identify populations at risk for iodine deficiency, there is currently no laboratory test that can determine the iodine status of an individual. Thus, it is recommended that all U.S. women who are pregnant, breastfeeding, or planning a pregnancy take a prenatal multivitamin that includes 150 µg (micrograms) of iodine daily.