Effect of thyroid hormone level on temperament in infants with congenital hypothyroidism detected by screening of neonates.

To determine the effect of congenital hypothyroidism and its treatment on infant behavior, we assessed temperament in 50 six-month-old infants with congenital hypothyroidism detected by means of screening of neonates. Intelligence and temperament were also evaluated at 12, 18, 24, and 36 months. More of these children were classified as "difficult" than children in the nonhypothyroid standardization sample. Temperamental difficulty was associated with increased nervous system sensitivity, reflecting more intense responses and a lower threshold of response to external stimulation. Greater temperamental difficulty was found to persist until at least age 2 years of age and to be associated with higher circulating triiodothyronine and thyroxine levels between 1 and 3 months of age. Our results suggest that behavioral features should be considered, as well as circulating hormone levels, in determining the proper dose of thyroid hormone replacement in infants with congenital hypothyroidism.