The effect of tobacco exposure on maternal and fetal thyroid function.

Objective: To determine the effect of smoking on maternal and fetal thyroid function.

Study Design: This prospective cohort study involved healthy women undergoing elective cesarean section for term singleton infants. Maternal and fetal thyroid indices were compared between smokers (n=21) and non-smokers (n=83).

Results: Maternal thyrotropin (TSH) concentrations were significantly lower in smokers than non-smokers (1.75 mIU/L versus 2.15 mIU/L, respectively, p=0.007), with similar free thyroxine (FT4) concentrations (9.59 pmol/L versus 9.56 pmol/L, p=0.755). For women who smoked, the correlation between the average number of cigarettes smoked per day and maternal TSH concentrations was -0.427, p=0.054. Infants of smokers and non-smokers had similar umbilical TSH (5.43 mIU/L versus 5.82 mIU/L, p=0.124) and FT4 concentrations (13.06 pmol/L versus 13.57 pmol/L, p=0.049).

Conclusion: We demonstrated for the first time that women who smoke during pregnancy have significantly lower TSH concentrations than non-smokers.