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.