Congenital Hypothyroidism: Influence of Disease Severity and L-Thyroxine Treatment on Intellectual, Motor, and School-Associated Outcomes in Young Adults

Objective. To describe intellectual, mo- tor, and school-associated outcome in young adults with early treated congenital hypothyroidism (CH) and to study the association between long-term outcome and CH variables acting at different points in time during early development (CH severity and early L-thyroxine treatment levels (0 - 6 years)). Methods. Neuropsychological tests were adminis- tered to all 49 subjects with CH identified during the first 3 years of the Norwegian neonatal screening program (1979 -1981) at a mean age of 20 years and to 41 sibling control subjects (mean age: 21 years). Results. The CH group attained significantly lower scores than control subjects on intellectual, motor, and school-associated tests (total IQ: 102.4 (standard devia- tion: 13) vs 111.4 (standard deviation: 13)). Twelve (24%) of the 49 CH subjects had not completed senior high school, in contrast to 6% of the control subjects. CH severity (pretreatment serum thyroxine (T4)) correlated primarily with motor tests, whereas early L-thyroxine treatment levels were related to verbal IQ and school- associated tests. In multiple regression analysis, initial L-thyroxine dose ( 0.32) and mean serum T4 level during the second year ( 0.48) predicted Verbal IQ, whereas mean serum T4 level during the second year ( 0.44) predicted Arithmetic. Conclusions. Long-term outcome revealed enduring cognitive and motor deficits in young adults with CH relative to control subjects. Verbal functions and Arith- metic were associated with L-thyroxine treatment vari- ables, suggesting that more optimal treatment might be possible. Motor outcome was associated with CH sever- ity, indicating a prenatal effect. Pediatrics 2003;112:923- 930; congenital hypothyroidism, thyroid hormone, thyrox- ine treatment, adult outcome, intelligence, achievement, motor function.