Untreated congenital hypothyroidism (CH) is known to cause impaired intellectual development and neuropsychological problems. The advent of neonatal screening for hypothyroidism in the 1970s allowed infants with low thyroxine (T4) and elevated thyroid-stimulating hormone (TSH) to be identified and treated before any clinical evidence of hypothyroidism was apparent. This study presented the intelligence quotient (IQ) outcomes for a cohort of infants found to be hypothyroid on newborn screening at a time when newborn screening programs were in their infancy.

To determine the impact of early thyroid hormone replacement on the intellectual development of infants identified as hypothyroid on routine newborn screening.

Retrospective, case-control study in 5 US states from 1976 to 1978.