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Cigarette Smoking during Pregnancy Is Associated with Alterations in Maternal and Fetal Thyroid Function

Department of Endocrinology (B.S., A.H., B.K., A.T.H., B.V.), Peninsula Medical School, Royal Devon and Exeter Hospital, Exeter EX2 5DW, United Kingdom; and Department of Endocrinology (M.B., R.W.B.), James Cook University Hospital, Middlesbrough TS4 3BW, United Kingdom

Address all correspondence and requests for reprints to: Dr. B. Vaidya, Department of Endocrinology, Royal Devon, Exeter Hospital, Exeter EX2 5DW, United Kingdom. E-mail: bijay.vaidya{at}pms.ac.uk.

Context: Studies in the general population have shown lower serum TSH levels in smokers as compared with nonsmokers.

Aim: Our aim was to examine whether smoking is associated with changes in thyroid function of pregnant women and their fetus.

Subjects and Methods: We examined the relationship between smoking and thyroid function (serum TSH, free T₄, and free T₃) in two independent cohorts of pregnant women without a history of thyroid disorder or an overt biochemical thyroid dysfunction: 1) first-trimester cohort (median gestation 9 wk) (n = 1428) and 2) third-trimester cohort (gestation 28 wk) (n = 927). We also analyzed 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 with nonsmokers, median serum TSH was lower (first-trimester cohort: 1.02 vs. 1.17 mIU/liter, P = 0.001; third-trimester cohort: 1.72 vs. 1.90 mIU/liter, P = 0.037), and median serum FT₃ was higher (first-trimester cohort: 5.1 vs. 4.9 pmol/liter, P < 0.0001; third-trimester cohort: 4.4 vs. 4.1 pmol/liter, P < 0.0001). In both cohorts, serum FT₄ in smokers and nonsmokers were similar. The prevalence of anti-thyroperoxidase antibodies was also similar in smokers and nonsmokers in both cohorts. Cord serum TSH of babies born to smokers was lower than of those born to nonsmokers (6.7 vs. 8.1 mIU/liter, 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.