Smoking and early pregnancy thyroid hormone and antibody levels in euthyroid mothers of Northern Finland Birth Cohort 1986.

TitleSmoking and early pregnancy thyroid hormone and antibody levels in euthyroid mothers of Northern Finland Birth Cohort 1986.
Publication TypeJournal Article
Year of Publication2012
AuthorsMännistö T, Hartikainen A-L, Vääräsmäki M, Bloigu A, Surcel H-M, Pouta A, Järvelin M-R, Ruokonen A, Suvanto E
Date Published2012 Jun 12

Background: Smokers in the general population have lower thyrotropin (TSH) and higher free triiodothyronine (fT3) and free thyroxine (fT4) concentrations, but the results in pregnant population vary from no effect to decrease in TSH and fT4 concentrations and increase in fT3 levels. Our objective was to evaluate further the association of smoking before and during pregnancy with maternal thyroid function during pregnancy and with subsequent hypothyroidism. Methods: Our study population was a prospective population-based cohort (N=9362), the Northern Finland Birth Cohort 1986, with extensive data throughout gestation. The mothers underwent serum sampling in early pregnancy. The samples were assayed for TSH, fT3, fT4, thyroid-peroxidase and thyroglobulin antibodies (N=5805). Mothers with thyroid dysfunction diagnosed before or during pregnancy were excluded, rendering 4837 euthyroid mothers. Smoking status of mothers and fathers were requested by questionnaires during pregnancy. Subsequent maternal morbidity to hypothyroidism 20 years after the index pregnancy was evaluated using national registers. Results: Euthyroid mothers who smoked before or continued smoking during first trimester of pregnancy had higher serum fT3 (p<0.001) and lower fT4 (p=0.023) concentrations than nonsmokers. Smoking in the second trimester was associated with higher fT3 (p<0.001) concentrations, but no difference in fT4 concentrations compared with nonsmokers. Thyroglobulin antibodies were less common among smoking than nonsmoking mothers (2.5% vs. 4.7%, p<0.001), but the prevalence of thyroid-peroxidase antibodies was similar. Paternal smoking had no independent effect on maternal early pregnancy thyroid hormone or antibody concentrations. The risk of subsequent maternal hypothyroidism after follow-up of 20 years was similar among prepregnancy smokers and nonsmokers. Conclusions: In euthyroid women smoking during pregnancy was associated with higher fT3 levels and lower fT4 levels; possibly reflecting smoking-induced changes in peripheral metabolism of thyroid hormones. No differences were found in TSH concentrations between smokers and nonsmokers. Our results differ from those of the general population, which usually have shown smoking-induced thyroidal stimulation. This is possibly due to pregnancy-induced changes in thyroid function. Decreases in fT4 levels among smokers might predispose to hypothyroidism or hypothyroxinemia during pregnancy. Despite these changes in thyroid function, smoking did not increase the woman's risk of subsequent hypothyroidism.

PubMed ID22690922