TOPIC: Thyroid function in pregnancy
Title: Cigarette smoking during pregnancy is associated with alterations in maternal and fetal thyroid function.
Authors: Shields B, Hill A, Bilous M, Knight B, Hattersley AT, Bilous RW, & Vaidya B.
Reference: Journal of Clinical Endocrinology & Metabolism 94: 570-574, 2009
Studies in the general population have shown lower serum TSH levels in smokers as compared with non smokers.
To examine whether smoking is associated with changes in thyroid function of pregnant women and their fetus.
Methods & Main Outcome Measures
The relationship between smoking and thyroid function (characterized by serum TSH, free T4 and free T3) was examined in (1) a 1st trimester cohort (N = 1.428; median gestation age: 9 weeks), (2) a 3rd trimester cohort (N = 927; median gestational age: 28 weeks) and (3) thyroid hormone levels in the cord serum of 618 full-term babies born to the women in the 3rd trimester cohort. The main outcome measures were thyroid function tests and prevalence of thyroid antibodies.
The median serum TSH was significantly lower in smokers in the 1st & 3rd trimesters than in non smokers (1.02 versus 1.17, and 1.72 versus 1.90 mU/L, respectively). Median serum free T3 was significantly higher in smokers in both trimesters compared to non smokers (5.1 versus 4.9 and 4.4 versus 4.1 pmol/L). There was no difference in free T4 or the prevalence of thyroid peroxidase autoantibodies in smokers and non smokers in both cohorts. Cord serum TSH of babies born to smokers was significantly lower than of those born to non smokers (6.7 versus 8.1mU/L).
Cigarette smoking is associated with changes in maternal thyroid function throughout pregnancy and also in fetal thyroid function, as measured in cord blood samples.
Previous studies in non pregnant individuals have shown lower TSH and higher free T3 levels in smokers compared to non smokers and some studies have also shown higher FT4 levels. The cross sectional data reported here confirm these findings in pregnant women and even show changes in the newborn cord serum. It should be noted that cigarette consumption was assessed by questionnaire analysis and quantitative analysis of smoking (eg cotinine) was not performed. Two questions are relevant. What is the mechanism of the smoking related changes and are they actually important in gestation?
Smoking may possibly alter thyroid function by sympathetic stimulation, by the direct antithyroid action of thiocyanate (a constituent of cigarette smoke), by an association with a reduced level of thyroid peroxidase autoantibodies and by an effect on deiodinase activity. The present study does not precisely clarify the mechanisms of cigarette smoking on thyroid function, except to indicate that the immune action is unlikely as antibody prevalence did not differ between the groups. However, it is possible that the effect of smoking on an increase in the type 2 deiodinase activity could account for the increase in serum free T3 in maternal serum.
There is agreement that changes in maternal function during gestation can affect fetal and childhood outcomes. Smoking is known to adversely affect fetal outcome, probably not related to thyroid function, although this has not been proven. Although the changes reported in this study were small, the results point the way to further examination of factors including smoking during gestation which may alter thyroid function and potentially affect both the mother and her fetus.
Summary and Commentary prepared by John Lazarus (Related to Chapter 14 of TDM)