Fig. 1

Flowchart of mother-child pairs identification. A total of 422,851 mother-child pairs were captured from 2001 to 2015. A total of 502 perinatal death cases and 193 mother-child pairs with missing gestational age and birth weight were excluded. Mothers exposed to anti-thyroid drugs, underwent thyroidectomy or radioactive iodine therapy during the study period, and mothers first exposed to L-T4 after giving birth were also excluded. The final cohort included 401,207 mother-child pairs. There were 2125 children born from mothers who received L-T4 during pregnancy, and 398,909 children born from mothers who had no history of thyroid-related diagnoses or prescriptions. There were 173 children born from mothers exposed to L-T4 before pregnancy but stopped the treatment when pregnant