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.