Too Much Thyroid Hormone Can Harm Fetus

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

From the WebMD Archives

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.

The couples' miscarriage rates were as follows:

  • If the pregnancy had an affected mother there were more miscarriages -- 23% more, compared with 2% for pregnancies where the father was affected and 4% in pregnancies with unaffected mothers.
  • Babies (without thyroid hormone resistance) born to affected mothers (those with high levels of thyroid hormone) were significantly smaller than babies born to unaffected mothers. Because of the mothers' high levels of thyroid hormone, newborn infants with normal thyroid systems responded by not making thyroid hormone of their own. Within a few weeks of life, they started to make their own thyroid hormone.
  • Unaffected mothers had normal rates of miscarriages and deliveries; they gave birth to equal numbers of affected and unaffected babies. "Normal miscarriages" for the general population is 8%, he adds.

His data shows that high levels of this hormone "can exert a direct toxic effect on fetal development," writes Refetoff. "It is important to recognize that overreplacement appears to be ... detrimental."

Don't Panic, but See a Doctor

Ellen Seely, MD, director of the pregnancy-related endocrine disorders clinic at Brigham-Women's Hospital in Boston, takes issue with Refetoff's miscarriage statistics.

The women in his study were likely being watched by doctors more closely. So any very early miscarriages -- when a woman simply gets her period -- may have been documented, resulting in higher numbers, she explains.

In the U.S., overall miscarriage rates are nearly as high as the 23% that Refetoff shows for the affected women in his study, Seely tells WebMD.

Nevertheless, thyroid problems are important "but manageable," Seely says. "We don't want to panic people. Women who have thyroid problems and are planning to get pregnant should discuss changes in thyroid hormone dose with their doctor."

If you find yourself pregnant and haven't planned it, get a blood test right away, she advises. The test is called the thyroid-stimulating hormone (TSH) test. "It usually has a turnaround of 24 hours. Then you can get a dosage adjustment made."

Most miscarriages are caused by chromosomal defects -- not by factors such as hormone levels, points out Ashi Daftary, MD, professor of maternal and fetal medicine at the University of Pittsburgh School of Medicine.

"Most women taking thyroid medicine won't require a change during pregnancy," he tells WebMD. "Instead of making a 30% adjustment in her medication -- based on the mere fact that she's pregnant -- her doctor should follow her very closely very early so appropriate adjustments can be made. A sizeable number of women taking thyroid medication will not need any increase in dosage."

Show Sources

SOURCES: Anselmo, J. Journal of the American Medical Association, Aug. 11, 2004; vol 292: pp 691-695. Samuel Refetoff, MD, with the Genetics and Molecular Medicine department at University of Chicago. Ellen Seely, MD, director, pregnancy-related endocrine disorders clinic, Brigham-Women's Hospital, Boston. Ashi Daftary, MD, professor of maternal and fetal medicine, University of Pittsburgh School of Medicine. WebMD Medical News: "Taking Thyroid Hormone: Take More if Pregnant."
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