HEALTH NEWS

Study Title:

Pregnancy and Smoking: Thyroid Consequences

Study Abstract

Context: Studies in the general population have shown lower serum thyrotropin (TSH) levels in smokers as compared to non-smokers.

Aim: To examine whether smoking is associated with changes in thyroid function of pregnant women and their fetus.

Subjects & methods: We examined the relationship between smoking and thyroid function (serum TSH, FT4 and FT3) in two independent cohorts of pregnant women without a history of thyroid disorder or an overt biochemical thyroid dysfunction: (a) first trimester cohort (median gestation 9 weeks) (n=1428), and (b) third trimester cohort (gestation 28 weeks) (n=927). We also analysed the relationship between maternal smoking and thyroid hormone levels in cord serum of 618 full-term babies born to the women in the third trimester cohort.

Results: In smokers compared to non-smokers, median serum TSH was lower (first trimester cohort: 1.02 mIU/l v 1.17 mIUl/l, p=0.001; third trimester cohort: 1.72 mIU/l v 1.90 mIU/l, p=0.037) and median serum FT3 was higher (first trimester cohort: 5.1 pmol/l v 4.9 pmol/l, p<0.0001; third trimester cohort: 4.4 pmol/l v 4.1 pmol/l, p<0.0001). In both cohorts, serum FT4 in smokers and non-smokers were similar. The prevalence of anti-thyroperoxidase antibodies was also similar in smokers and non-smokers in both cohorts. Cord serum TSH of babies born to smokers was lower than of those born to non-smokers (6.7 mIU/l v 8.1 mIU/l; p=0.009).

Conclusions: Cigarette smoking is associated with changes in maternal thyroid function throughout the pregnancy and in fetal thyroid function as measured in cord blood samples.

From press release:

Cigarette smoking during pregnancy is associated with potentially harmful changes in both maternal and fetal thyroid function, according to a new study.

"We studied the influence of cigarette smoking on thyroid function of two groups of women at different stages of pregnancy – one in the first trimester and the other in the third trimester," said Dr. Bijay Vaidya, Ph.D., of Peninsula Medical School at Royal Devon and Exeter Hospital in the United Kingdom, and coauthor of the study. "In both groups we found that smoking during pregnancy is associated with changes in the mothers' thyroid hormone levels."

Optimal maternal thyroid function during pregnancy is vital for a successful pregnancy outcome, said Dr. Vaidya. The adverse outcomes associated with thyroid dysfunction during pregnancy include increased risk of miscarriage, premature birth, low birth weight and impaired neuropsychological development of the baby.

Dr. Vaidya and his colleagues also measured thyroid hormone levels in the umbilical cord of babies born to smoking mothers and found that smoking-related changes in thyroid function extend to the fetus. Dr. Vaidya believes that impaired thyroid function in the fetus could have potentially harmful biological consequences.

The study also found that in mothers who stopped smoking during pregnancy their thyroid hormone levels were comparable to levels found in non-smokers, which suggests that changes in thyroid function are rapidly reversible.

There is currently no definitive explanation for how smoking affects thyroid function, but Dr. Vaidya suggests that smoking may influence thyroid hormone levels by affecting the enzyme which converts the active form of thyroid hormone to an inactive form.

Other researchers working on the study include Beverley Shields, Anita Hill, Beatrice Knight, and Andrew Hattersley of Royal Devon and Exeter Hospital in Exeter, U.K., and Mary Bilous and Rudy Bilous of James Cook University Hospital in Middlesbrough, U.K.

Study Information

Beverley Shields, Anita Hill, Mary Bilous, Beatrice Knight, Andrew T Hattersley, Rudy W Bilous, and Bijay Vaidya.
Cigarette smoking during pregnancy is associated with alterations in maternal and fetal thyroid function.
Journal of Clinical Endocrinology & Metabolism
2008 November
Department of Endocrinology, Peninsula Medical School, Royal Devon & Exeter Hospital, Exeter; Department of Endocrinology, James Cook University Hospital, Middlesbrough, UK. .