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Smoking cigarettes linked to changes in maternal and fetal thyroid function
Those who stopped smoking during pregnancy had thyroid hormone levels similar to nonsmokers.
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Cigarette smoking was associated with lower maternal and fetal thyroid-stimulating hormone levels, as well as other changes in maternal and fetal thyroid function.
Researchers from the United Kingdom examined a first trimester group (n=1,428) and a third trimester group (n=927) of pregnant women without a history of thyroid disorder or overt biochemical thyroid dysfunction. They also evaluated the thyroid hormone levels in cord serum for full-term babies (n=618) born to women in the third trimester group.
Thyroid function impaired
For smokers, median serum TSH was 1.02 mIU/L in the first trimester group and 1.72 mIU/L in the third trimester group, according to the researchers. For nonsmokers, medium serum TSH was 1.17 mIU/L (P=.001) in the first trimester group and 1.90 mIU/L (P=.037) in the third trimester group.
Median serum-free triiodothyronine was 5.1 pmoL/L in the first trimester group and 4.4 pmoL/L in the third trimester group for smokers. For nonsmokers, free T3 was 4.9 pmoL/L in the first trimester group (P<.0001) and 4.1 pmoL/L in the third trimester group (P<.0001).
Serum-free thyroxine was similar in smokers and nonsmokers for the first and third trimester groups, according to the researchers. For smokers and nonsmokers in both groups, the prevalence of anti-thyroperoxidase antibodies was also similar.
Cord serum TSH for babies born to smokers was 6.7 mIU/L compared with 8.1 mIU/L for babies born to nonsmokers (P=.009). In both groups, women who stopped smoking during pregnancy had thyroid hormone levels similar to nonsmokers, according to the researchers.
“Although the magnitude of effect of smoking on thyroid function in our study was small, the smoking-related changes in the thyroid function extended to the fetus, suggesting these changes could have a biological impact on the fetus,” the researchers wrote. “Further studies are needed to analyze whether the changes in the maternal and fetal thyroid function associated with maternal smoking result in any adverse pregnancy outcomes.”
J Clin Endocrinol Metab. 2008;doi:10.1210/jc.2008-0380.