Endocrine Abstracts

Introduction: Pregnancy is a stress test on maternal thyroid and pregnant women may be more susceptible to thyroid dysfunction. Consequences of this may be harmful for the mother and fetus. Smoking has been associated with alterations in thyroid hormone concentrations, but little knowledge exists on this association in pregnant women.

Subjects and methods: The study population is large, prospective population-based Northern Finland Birth Cohort 1986 (n=9362), with extensive data throughout gestation. Mothers underwent serum sampling in early pregnancy. Samples have been assayed for TSH, free thyroxine (fT₄), free triiodothyronine (fT₃), thyroid-peroxidase and thyroglobulin antibodies (n=5805). Mothers with thyroid dysfunction diagnosed before or during pregnancy were excluded. Smoking status of mothers and fathers were asked by questionnaires during pregnancy. Subsequent maternal morbidity to hypothyroidism 20 years after the index pregnancy was evaluated using national registers.

Results: Mothers who smoked before pregnancy or during its first trimester had higher fT₃ (P<0.001) and lower fT₄ (P=0.023) concentrations than non-smokers. Smoking in second trimester was associated with lower TSH (P=0.021) and higher fT₃ (P<0.001) concentrations, but no difference in fT₄ concentrations compared to non-smokers. Cessation of smoking during pregnancy did not improve maternal thyroid hormone status. Thyroglobulin antibodies were less common among smoking than non-smoking mothers (2.5 vs 4.7% respectively, P<0.001), but the prevalence of thyroid-peroxidase antibodies was similar. Risk for subsequent hypothyroidism of the mother was similar among smokers and non-smokers. Paternal smoking did not have an independent effect on maternal early pregnancy thyroid hormone/antibody concentrations.

Discussion: Smoking during pregnancy was associated with alterations in thyroid hormone concentrations which may be due to smoking-induced changes in peripheral metabolism of thyroid hormones. Smoking may therefore render the diagnosis of thyroid dysfunction in pregnant women even more difficult. Along with other known health risks, smoking affects maternal thyroid function, providing more weight to cessation of smoking.