Thyroid hormone replacement therapy in pregnancy and motor function, communication skills, and behavior of preschool children: The Norwegian Mother, Father, and Child Cohort Study

Purpose Limited research has focused on the association between prenatal thyroid hormone replacement therapy (THRT) and motor function, communication skills, and behavior in preschool children. Here, we estimated the association between THRT during pregnancy and the first trimester and these developmental outcomes. Methods This study was based on the Norwegian Mother, Father, and Child Cohort Study (MoBa) and other national registries. We included mother–child pairs exposed to THRT during pregnancy (n = 663), after delivery (n = 728), or unexposed (n = 28 040). Exposure to THRT was defined according to filled prescriptions. Child outcomes, presented as T‐score differences, were parent‐reported using the Ages and Stages Questionnaire, Strengths and Difficulties Questionnaire, and Child Behavior Checklist. Results Of 29 431 mother–child pairs, 2.3% were prenatally exposed to THRT. We found no difference between prenatally exposed and unexposed children in regards to gross motor function (β: 0.17, 95% CI −1.19, 1.54), fine motor function (β: −0.17, 95% CI −1.14, 0.80), communication (β: −0.31, 95% CI −1.58, 0.96), externalizing (β: −0.03, 95% CI −1.07, 1.01), internalizing (β: 0.89, 95% CI −0.20, 1.97), or social behaviors (β: −0.04, 95% CI −0.92, 0.84). Somatic complaints were higher in THRT‐exposed children (β: 0.98, 95% CI 0.08, 1.87), and children whose mothers were exposed after delivery had more sleep problems than unexposed children (β: 0.99, 95% CI 0.24, 1.74). Conclusions Children prenatally exposed to THRT have developmental outcomes as positive as unexposed children on motor function, communication, and behavior. The association with somatic complaints and sleep were not clinically relevant.