Context
Thyroid hormones are critical for neural development, and during the first trimester of pregnancy, the fetus relies fully on maternal thyroid hormone production.

Objective
To investigate the associations between maternal thyroid hormone levels in the first trimester with the child's school performance, risk of attention deficit hyperactivity disorder (ADHD), and autism spectrum disorder (ASD).

Methods
Information from the Copenhagen Primary Care Laboratory Pregnancy Database on first trimester TSH and free thyroxine measurements in mothers of children born in 2000 through 2014 were linked with information on the child's standardized test scores in school, ADHD (patient record diagnoses and medication), and ASD (patient record diagnoses) until the end of 2018. Associations of TSH and free thyroxine with the outcomes were individually assessed by linear mixed models and Cox regression models. The analyses were stratified by preexisting maternal thyroid disorders.

Results
TSH measurements were available for 17 909 mother-child dyads. Among those with children born in 2000 through 2009, 6126 had a standardized school test score and were analyzed for the association between maternal thyroid hormone levels and child's school performance, and no support for an association was found. The association between thyroid hormone levels and child's risk of ADHD and ASD were analyzed for the 17 909 dyads and with no support for an association between thyroid hormone levels and these neurodevelopmental disorders. Stratification by preexisting maternal thyroid disorders did not affect the results.