Intellectual development in children with congenital hypothyroidism in relation to recommended thyroxine treatment.

Department of Pediatric Research, National Hospital, Oslo, Norway.

The relationship between the treatment serum thyroxine level and intellectual development at 2 and 6 years was investigated in 46 Norwegian children with congenital hypothyroidism identified by neonatal screening. The level of serum thyroxine during the first 2 years was positively correlated with the Mental Development Index at 2 years of age (Bayley Scales of Infant Development) and the Verbal IQ at 6 years of age (Wechsler Preschool and Primary Scale of Intelligence). Children with a mean serum thyroxine level greater than 180 nmol/L (14 micrograms/dl) during the first year had a significantly higher Mental Development Index at 2 years and Verbal IQ at 6 years than children with serum thyroxine values less than 129 nmol/L (10 micrograms/dl). Boys had a lower Mental Development Index at 2 years of age than girls (86.9 vs 105.1; p less than 0.001) and a higher frequency of elevated serum levels of thyroid-stimulating hormone during the first year (p = 0.001). No signs of toxic effects of a high hormone level at the time of IQ assessment were detected. However, high serum levels of thyroxine at ages 2 to 4 years in girls were related to lower Performance IQ at age 6 years. The results demonstrate that the serum level of thyroxine is of importance in relation to intellectual development. Thyroxine levels above the upper reference range during the first 2 years were related to best intellectual development at 2 and 6 years.

PMID: 2040919 [PubMed - indexed for MEDLINE]