updated 2/6/2007 12:29:55 PM ET 2007-02-06T17:29:55

Even a slightly underactive thyroid — too mild for symptoms — during pregnancy might trigger premature birth and babies born with lower IQs. But doctors don’t know if treating a symptom-free mother would help.

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Now the National Institutes of Health is beginning a major study of pregnant women to find out, a key question amid growing debate about whether more mothers-to-be should get their glands checked.

“It’s all up in the air,” cautions Dr. Catherine Spong, pregnancy chief at the NIH’s National Institute of Child Health and Human Development. “We don’t know that treatment is going to improve the outcome.”

About 27 million Americans have either an overactive or underactive thyroid, the bow-tie-shaped gland in the neck that produces hormones that regulate metabolism and stimulate almost every type of tissue. Only about half have been diagnosed, according to the American Association of Clinical Endocrinologists.

An overactive thyroid increases heart rate and blood pressure, and can cause weight loss, depression, confusion and vision problems.

An underactive thyroid, called hypothyroidism, is far more common, with even harder-to-recognize symptoms: fatigue, constipation, weight gain, forgetfulness, a hoarse voice, dry skin. A thyroid sluggish enough to cause symptoms is called “overt hypothyroidism,” and it increases the risk of heart disease, bone-thinning osteoporosis and infertility unless treated with a once-a-day hormone pill.

During pregnancy, having enough thyroid hormones is important for fetal brain development — especially during the first trimester, when the fetus depends solely on the mother for them — and for avoiding a miscarriage or premature birth. So mothers-to-be with overt hypothyroidism are supposed to be treated.

Pregnancy wrinkle
Here’s the pregnancy wrinkle: Mildly underactive thyroids may not provide fetuses enough of the hormones even if the mothers report no symptoms. And when thyroid hormone levels are normal, mothers’ bodies still may harbor immune system cells called antibodies that subtly attack the gland — antibodies also linked to prematurity and pregnancy loss, and that are more common in women with other autoimmune diseases such as Type 1 diabetes.

Among the worrisome evidence, a 1999 study found that untreated hypothyroidism increases the risk of having children with lowered IQs, whether the mother had thyroid symptoms or not. And in 2005, Texas researchers who tested more than 17,000 pregnant women found those with asymptomatic hypothyroidism had almost twice the risk of a premature baby as did women with normal thyroids.

Outcome of treatment unknown
But other studies have raised questions about the links, and no one yet knows if treatment would help, have no effect or, possible but unlikely, do harm. So while obstetricians are under increasing pressure to check thyroids, they’re reluctant to routinely test the nation’s 4 million-plus pregnant women to find the ones — up to one in 50 — who may have asymptomatic hypothyroidism.

Enter the NIH study. Hospitals in 11 states are beginning to screen thousands of otherwise healthy pregnant women to find 1,000 with asymptomatic thyroid problems. Half will get thyroid medication. Scientists will track their children’s brain development through age 5.

A similar study in Britain, which screened 22,000 pregnant women, is under way.

Last summer, Italian researchers reported the first hint that treatment might help. They tested 984 pregnant women, finding 12 percent who bore thyroid antibodies despite normal hormone levels. The half who received thyroid medication had no more miscarriages or premature deliveries than normal women — while those who weren’t treated had roughly triple the risk.

“This was the first really powerful piece of data showing that it (treatment) makes a difference,” says Dr. Alex Stagnaro-Green of the University of Medicine and Dentistry of New Jersey, an endocrinologist who specializes in pregnancy thyroid problems. “Everyone is waiting for the next study to confirm or refute.”

One study isn’t proof, agrees Dr. Diane Ashton of the March of Dimes, which is monitoring the research as part of its campaign to lower the nation’s half a million premature births each year.

So until the issue is settled, what’s the advice?

  • All sides agree that anyone with thyroid disease’s vague symptoms should be tested, and treated if they have overt disease.
  • An international endocrinology panel soon will issue guidelines expected also to urge testing for pregnant women at high risk because of thyroid disease in the family, or if they have Type 1 diabetes or other autoimmune diseases. Stagnaro-Green adds testing of both hormone levels and antibodies for women who already have had a miscarriage or premature baby.
  • Women already diagnosed with hypothyroidism probably will need a significantly higher dose of thyroid hormone during pregnancy.
  • Use prenatal vitamins containing iodine, important for proper thyroid function.

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