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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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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.

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