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

Abstract

Objective. To describe intellectual, motor, 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 administered 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 deviation: 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 (β = 0.32) and mean serum T4 level during the second year (β = 0.48) predicted Verbal IQ, whereas mean serum T4 level during the second year (β = 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 Arithmetic were associated with l-thyroxine treatment variables, suggesting that more optimal treatment might be possible. Motor outcome was associated with CH severity, indicating a prenatal effect.