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Excess thyroid hormone harmful to fetus

Researchers from the University of Chicago have showed, for the first time, that an excess of thyroid hormone during pregnancy has a direct toxic effect on the fetus, tripling the risk of miscarriage and reducing the average birth weight of infants who survive.

The problems caused by a lack of sufficient thyroid hormone during pregnancy are well known and approximately two percent of all pregnant women now take supplements to prevent a shortage. A recent article in the New England Journal of Medicine proposed that all women being treated prior to conception for low thyroid hormone -- a common occurrence -- "increase the dose by 29 percent" as soon as they learn they are pregnant.

The University of Chicago study, however, published in the 11 August 2004 issue of JAMA, shows that increasing the dose without careful testing is hazardous. Overcompensating for a mother's shortage of thyroid hormone, the authors note, can be equally harmful.

"Although the risks of insufficient thyroid hormone during pregnancy are established," said study author Samuel Refetoff, M.D., professor of medicine and pediatrics and the committees on genetics and molecular medicine at the University of Chicago, "we have not, until now, been able to determine the consequences of an excess. We now see that having too much is just as bad as having too little. This tells us that hormonal replacement must be assessed and fine-tuned so as not to exceed the normal requirements."

Physicians have not been able to study the effect on the fetus of excess thyroid hormone, which regulates metabolism, because it was impossible to separate the effects on the fetus from the effects on the mother.

This innovative study, however, provides a perfect example of how "errors of nature can help us understand normal hormone action," said Refetoff.

Nearly 40 years ago, he found such an error. In 1967, Refetoff described a clinical syndrome involving resistance to thyroi
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Contact: John Easton
jeaston@uchospitals.edu
773-702-6241
University of Chicago Medical Center
10-Aug-2004


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