The Effect of Tobacco Exposure on Maternal and Fetal Thyroid Function

ABSTRACT Smoking has adverse effects on thyroid function in nonpregnant women. For example, a previous study found a small but statistically significant decrease in the concentrations of thyrotropin (TSH) in smokers compared to nonsmokers, and a recent meta-analysis found that the risk of Graves’ disease was three-fold greater among smokers than in those who had never smoked; cessation of smoking decreased the risk. However, data on the effects of smoking during pregnancy on maternal and neonatal thyroid function have been conflicting. This prospective cohort study evaluated the effect of smoking on maternal and neonatal thyroid function in 104 healthy women (83 nonsmokers and 21 smokers) undergoing elective cesarean section for term singleton infants during the years 2002–2004. The smoker to nonsmoker ratio (4:1) reflected the general rates of smoking during pregnancy. Mothers who smoked were significantly younger than nonsmokers (median age 29.5 versus 33.0 years, respectively, P ≤ .023), and had significantly lighter infants (3250 gm versus 3456 gm, respectively, P ≤ .011). Smokers had significantly lower maternal TSH concentrations than nonsmokers (median 1.75 mIU/liter versus 2.15 mIU/liter, respectively, P ≤ .007), but there was no difference in free thyroxine (FT4) concentrations (median 9.59 pmol/liter versus 9.56 pmol/liter (P = .755). There was a correlation between the number of cigarettes smoked per day and the maternal TSH concentrations (R = –.427; P ≤ .05). Umbilical venous TSH and FT4 concentrations were similar in infants of nonsmokers and smokers (P = .049). This is the first study to show that women who smoke during pregnancy have significantly lower TSH concentrations than nonsmokers.