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Taking Thyroid Hormone? Take More if Pregnant

Extra Weekly Levothyroxine Doses Advised When Pregnancy Confirmed
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July 14, 2004 -- As soon as they know they're pregnant, women taking thyroid hormone should up their dosage.

The advice comes from a study of 19 women taking thyroid hormone -- levothyroxine -- for thyroid deficiency. The women's need for thyroid hormone increased during the first weeks of pregnancy.

That's important. During these weeks, the developing fetus is totally dependent on the mother's supply of thyroid hormone. Too little, and the baby risks impaired mental development and even death, note Erik K. Alexander, MD, of Boston's Brigham and Women's Hospital, and colleagues. They researchers report their findings in the July 15 issue of The New England Journal of Medicine.

"We suggest that women with hypothyroidism be instructed to increase their usual levothyroxine intake by two additional doses each week immediately on confirmation of pregnancy," Alexander and colleagues write. "[They should] contact their health care provider so that a program of test-guided dose adjustments can be instituted."

Alexander's team found that most of the women needed extra thyroid hormone starting at about eight weeks into their pregnancies. Most women don't see their obstetrician until the 10th week of pregnancy. By this time, most of the women in the study would have thyroid levels dangerously low for their developing baby.

In an editorial accompanying the study, Anthony Toft, MD, of the Royal Infirmary in Edinburgh, Scotland, agrees that increasing the dose of thyroid hormone would be helpful. He suggests, however, that it might be simpler to increase the daily dose of thyroid hormone by 25 to 50 micrograms a day until tests can determine an individual woman's exact need.

Toft also suggests that women of childbearing age should be tested for thyroid deficiency. That might well be wise. Alexander and colleagues note that every year, at least 12,000 to 16,000 U.S. children are born to mothers with improperly treated -- or unknown -- thyroid problems.

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