updated 5/11/2004 3:44:40 PM ET 2004-05-11T19:44:40

Even a mildly underactive thyroid — too mild for symptoms — may cause serious problems during pregnancy, such as premature birth or babies born with lower IQs.

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Yet whether to test every pregnant woman’s blood for thyroid deficiency is controversial.

Now specialists have come up with a compromise: a call to test women who are at high risk for thyroid disease, before they conceive or when they are in very early pregnancy, while pushing for major studies to settle whether even more should be checked.

Who’s at high risk?
Women who have thyroid disease in the family, or who themselves have a history of thyroid problems or other autoimmune disorders such as Type 1 diabetes, rheumatoid arthritis or lupus, says the American Thyroid Association.

“There are a lot of women being missed currently who would fall in these categories,” says Dr. Gregory Brent of the University of California, Los Angeles, who helped lead an ATA-called meeting last month on the issue.

The thyroid, a bow tie-shaped gland in the neck, produces hormones that regulate metabolism and stimulate almost every type of tissue.

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

Far more common is an underactive thyroid — hypothyroidism — that can slow body functions. Symptoms are often vague: fatigue, weight gain, depression, forgetfulness, a hoarse voice, dry skin, mood swings, intolerance to cold.

Overt thyroid disease increases the risk of heart disease, bone-thinning osteoporosis and infertility. Fortunately, it’s easily treated, and people with symptoms are supposed to get a simple $25 blood test for diagnosis.

But some people have a mildly underactive thyroid that hasn’t yet caused symptoms — only blood testing can detect it — and that’s the crux of the pregnancy debate.

Lowered IQs
A 1999 study found that untreated hypothyroidism in pregnancy increases the risk of having children with lowered IQs — even if the mother has no symptoms. Thyroid hormones are important for brain development, and in the first trimester, the fetus depends solely on the mother for them.

Now a new study shows those asymptomatic women are almost twice as likely to have premature babies.

Researchers at the University of Texas Southwestern Medical Center tested more than 17,000 women seeking routine prenatal care, and found 4 percent of the 404 with asymptomatic hypothyroidism delivered prematurely — compared with 2.5 percent of women with normal thyroids. Prematurity might explain the lower-IQ link, says lead researcher Dr. Brian Casey, who presented his results at last month’s thyroid meeting.

Obstetricians say there isn’t enough evidence yet to warrant testing all 4 million-plus pregnant women each year to find the roughly 2.5 percent thought to have asymptomatic hypothyroidism.

No one yet knows if treating those women reduces the risks, Casey stresses. Because crucial brain development occurs so soon after conception, “identifying women in pregnancy may be identifying them too late to have an impact,” he cautions.

Plus, hormone levels fluctuate during pregnancy, so there’s even debate about how to analyze women’s tests.

But with potentially thousands affected, the American Thyroid Association is pushing the government to fund such research now. Already, British scientists have begun testing 22,000 pregnant women to see if the resulting children of those treated for asymptomatic thyroid deficiency have better brain function. Results will take years; checking an impact on premature birth could be done more quickly.

Meanwhile, in addition to preconception or early pregnancy thyroid testing for high-risk women, the ATA advises:

—Women already diagnosed with hypothyroidism need additional testing during pregnancy, because they may need a 30- to 50-percent higher dose of thyroid hormone.

—Pregnant women should ensure their prenatal vitamins contain iodine, important for proper thyroid function. New research shows about a third of prenatal vitamins contain no iodine, and few of the rest contain a full dose. Pregnant and nursing women need 220 to 290 micrograms a day, more than the 150 required for other adults.

Most Americans get enough iodine, which is commonly added to salt, but studies suggest intake is dropping and no one knows how much pregnant women get. Iodine also is found in seafood, dairy products and bread.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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