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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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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."

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