Smoking reduces the risk of hypothyroidism and increases the risk of hyperthyroidism: evidence from 450 842 mothers giving birth in Denmark

ObjectiveSmoking may influence on the occurrence of thyroid disease, but studies have led to inconsistent results. In Denmark, information on maternal smoking during pregnancy is registered by midwives, and we investigated the association between maternal smoking as reported during pregnancy and the subsequent maternal risk of having hyper- or hypothyroidism diagnosed. DesignPopulation-based cohort study. ParticipantsUsing Danish nationwide registers, we identified mothers giving birth in Denmark, 1996-2008, and studied their first pregnancy in the study period. MeasurementsInformation on maternal smoking during the pregnancy and maternal diagnosis of hyper- or hypothyroidism was obtained from the Danish National Hospital Register (DNHR) and prescription of thyroid medication from the Danish National Prescription Register (DNPR). Cox proportional hazards model was used to estimate hazard ratio (HR) with 95% confidence interval (95% CI) for onset of maternal hyper- or hypothyroidism after birth of the child in multivariate analyses adjusting for potential confounders. ResultsAmong mothers included (n=450842), altogether 89022 (197%) reported that they were smokers during the first pregnancy in the study period, and 8905 (20%) developed hyper (n=3389)- or hypothyroidism (n=5516) after birth of the child. Maternal smoking was associated with a subsequent decreased risk of developing hypothyroidism (adjusted HR 075 (95% CI 070-081)) and an increased risk of hyperthyroidism (138 (127-149)). ConclusionsDanish nationwide registration of maternal smoking during pregnancy adds further evidence to an association between smoking and thyroid dysfunction; smoking reduced the risk of hypothyroidism and increased the risk of hyperthyroidism.