Methods: An existing database, involving a large cohort of children with congenital hypothyroidism detected by neonatal screening, was analyzed retrospectively, There were 94 children: 89 were assessed at age 7 years for intelligence and selective cognitive abilities, and 87 at age 8 years for behavior, achievement, and selective abilities.

Results: Subjects were stratified by median spilt into low and high starting dose groups. The high-dose group performed better on indexes of intelligence, verbal ability, and memory but had more behavior problems reflecting increased anxiety, social withdrawal, and poorer concentration. The dose of L-thyroxine at age 8 years was negatively correlated with memory task performance.

Conclusion: A higher starting dose of L-thyroxine is beneficial for subsequent intellectual outcome in children with congenital hypothyroidism but may be associated with internalizing behavior problems.