Q&A: Should Pregnant Women Have Tests for Abnormal Thyroid Function?

Q. What thyroid abnormalities in pregnancy might adversely affect the mother or fetus?

1. An underactive thyroid gland causes a condition referred to as hypothyroidism.

  • "Overt" hypothyroidism is associated with symptoms, such as fatigue, weight gain, dry skin, cold intolerance, constipation, and forgetfulness, which are often subtle and hard to recognize. Whether pregnant or not, women with overt hypothyroidism are treated with a daily pill of thyroid hormone to overcome symptoms and to prevent coronary artery disease and other disorders.
  • A more common problem is a mildly underactive thyroid gland that causes no identifiable symptoms - referred to as subclinical hypothyroidism - despite lower-than-normal levels of thyroid hormones in the body.

2. A larger number of pregnant women have abnormal levels of antibodies against the thyroid.

According to available studies, between 10 percent and 20 percent of pregnant women have these thyroid antibodies and 2 percent to 5 percent have undiagnosed, mostly subclinical, hypothyroidism.

3. Pregnant women may also exhibit overactivity of the thyroid gland (hyperthyroidism). Overt hyperthyroidism is usually evident from symptoms and must be treated. Subclinical hyperthyroidism is far less common than hypothyroidism in pregnant women.

Q. What potentially harmful effects can thyroid hormone abnormalities cause during pregnancy? 

Both thyroid antibodies and hypothyroidism increase the risk of miscarriages and preterm deliveries. A 1999 publication found that the children of mothers whose hypothyroidism was not treated during pregnancy had significantly lower IQ scores (including 19 percent with an IQ less than 85) than did the children of mothers with normal thyroid function during pregnancy.

Q. What are the options for screening pregnant women for thyroid abnormalities?

  • Don't screen at all. This option seems unreasonable.
  • Screen all pregnant women. Several endocrine and thyroid professional associations have advocated aggressively screening all pregnant women for thyroid abnormalities.
  • Limit screening to women who are at high risk for thyroid abnormalities during pregnancy. High-risk women include those who have possible symptoms of hypothyroidism, a history of thyroid disease in their family, type 1 diabetes or other autoimmune disorders, and possibly those with a history of a previous miscarriage or premature birth.

But such selective screening is problematic. A recent study shows that testing only high-risk pregnant women for hypothyroidism means passing over many women with subclinical thyroid abnormalities, who also need treatment.

In this study of pregnant women, 27 percent were found to be at high risk for thyroid abnormalities, and these women had a seven times greater incidence of abnormal thyroid tests than did the low-risk women. So far, so good. But fully one-third of the women in the study with hypothyroidism were not high-risk and so their mostly subclinical condition would not have been identified or treated.

Q. Are there benefits to treating thyroid abnormalities during pregnancy?

There is no clear-cut answer to this question. I have mentioned one study that seemed to show a lower intelligence among children if their mother's hypothyroidism during pregnancy was not treated with thyroid hormone. This conclusion is uncertain, however, because so few women participated in the study.

In an effort to obtain more meaningful information about this issue, the National Institutes of Health has initiated a study in 11 hospitals around the country. During their pregnancy, women with hypothyroidism will randomly either be treated with thyroid hormone or not. The IQs of their children will then be tested five years later.

One other study found that administration of thyroid hormone to women with thyroid antibodies reduced the number of miscarriages and preterm deliveries.

Q. What do I recommend?

I cannot make a general recommendation on whether all or only some pregnant women should undergo the expense of thyroid tests. In addition to the uncertainties already mentioned, there are differences of opinion on which tests should be done. But ... if one of my daughters told me she was pregnant, I would tell her that she could indeed afford the costs of the thyroid tests and should have them done.

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