The purpose of this study was to assess how much of the variance in intellectual outcome at 2 and 6 years of age could be attributed to treatment variables in children with congenital hypothyroidism, and which of the parameters used for monitoring treatment predicted later development. Forty-five children, early treated according to general recommendations, were studied. Linear multiple regression analysis was used, controlling for socio-economic status and the pretreatment serum thyroxine concentration. At 2 years of age, 19% of the variance in Mental Development Index (Bayley Scales of Infant Development) was attributed to treatment variables: combinations of serum thyroxine and serum TSH during the 1st year and bone age at mean age 1.5 years (mean z-scores). At 6 years of age, 35% of the variance in Verbal IQ (Wechsler Preschool and Primary Scale of Intelligence) was attributed to treatment variables: 13% to the mean serum thyroxine concentration during the 1st year, 12% to the initial L-thyroxine dose per kilogram body weight per day, and 10% to a combined measure for serum thyroxine and serum TSH during the 2nd year (mean z-score).
Conclusion: Both the initial L-thyroxine dosage and treatment variables during the 1st and the 2nd year (serum thyroxine, serum TSH and bone age) predicted later intellectual outcome in children with congenital hypothyroidism.