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Too Much Thyroid Hormone Can Harm Fetus

<P>Miscarriage Risk May Be Higher, but Don't Panic -- It's Manageable</P>
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WebMD Health News

Aug. 10, 2004 -- High levels of thyroid hormones can have a direct toxic effect on fetal development, a new study shows. Women with thyroid problems should see their doctors -- and get a blood test -- right away, researchers say.

"Our data show a threefold to fourfold increase in the rate of miscarriage" in mothers with excess thyroid hormones, writes researcher Samuel Refetoff, MD, with the genetics and molecular medicine department at University of Chicago.

His paper appears in this week's issue of the Journal of the American Medical Association (JAMA). It comes on the heels of evidence published just a few weeks ago -- showing that women with thyroid deficiency should increase their dosage each week when they learn they are pregnant until tests can be done to determine their exact needs.

There's much at stake: During those early weeks, the developing fetus is totally dependent on the mother's supply of thyroid hormone. Too little, and the risks to the baby include impaired mental development and even death. Because the impact on babies is so serious, newborns are routinely screened for this deficiency.

A woman's need for thyroid hormone increases during the first weeks of pregnancy; some 2% of pregnant women take supplements to prevent this deficiency.

However, the consequences of excess thyroid hormone -- for the baby -- are not well known. Studying this interaction has been difficult. "It's not been clear whether the problem during pregnancy is caused by the overactivity of the mother's body functions (because of hyperthyroidism) -- or because they are giving too much hormone to the baby," Refetoff tells WebMD. "It is impossible to dissociate one from the other."

His study is the first to shed some light on this issue. "We're finding that excess hormone is as bad, and probably worse, than too little," says Refetoff. "Prescribing these hormones without testing the mother first is not wise."

Portuguese Family Helps Shed Light

In their study, Refetoff and his colleagues focused on a unique family (native to Portugal) with an inherited syndrome involving thyroid hormone. It's known as thyroid hormone resistance.

Those who get this mutation produce more thyroid hormone than normal, he explains. However, the excess is normal for them -- so they don't have increased metabolism, heart rate, and other problems usually caused by excess hormone levels.

For women inheriting this genetic mutation, pregnancy can be problematic. If her baby does not inherit the mutation, her excess thyroid hormones will be excessive for the fetus, Refetoff explains. This group of women "represents a very unique opportunity" to study this problem, he says.

His research group analyzed medical records for 167 members of this family, including 36 couples. They compared pregnancies of "affected mothers" or "affected fathers" (those with the mutation) against those without the mutation -- looking at miscarriage rates, and at newborns' birth weights and thyroid hormone levels.

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