The effect of neonatal hypothyroidism and low family income on intellectual disability: A population-based cohort study.

Background: To investigate relationships among neonatal hypothyroidism, family income, and intellectual disability, as well as the combined effects of neonatal hypothyroidism and low family income on intellectual disability.

Methods: Data were extracted from the National Health Insurance Service-National Sample Cohort from 2002 to 2011. This retrospective study included 91,247 infants. The presence of intellectual disability was based on the disability evaluation system in Korea. Newborn hypothyroidism was identified from diagnosis and prescription codes. Family income was determined from average monthly insurance premiums. Cox proportional hazards models were used to calculate adjusted hazard ratios.

Results: Of the 91,247 infants, 208 were considered to have intellectual disability (29.18 cases per 100,000 person-year). The risk of intellectual disability was higher in infants with hypothyroidism than in those without hypothyroidism (hazard ratio = 5.28, P: < .0001). The risk of intellectual disability was higher in infants with low family income than in those with high family income (hazard ratio = 2.32, P: < .0001). The risk of intellectual disability was higher in infants with hypothyroidism and low family income than in those without hypothyroidism and with high family income (hazard ratio = 36.05, P: < .0001).

Conclusions: Neonatal hypothyroidism and low family income were associated with the risk of intellectual disability in Korea. Additionally, neonatal hypothyroidism and low family income significantly increased the risk of intellectual disability. Public health policymakers should consider providing additional resources for alleviating neonatal hypothyroidism among low-income families.