Abstract:
Background: While severe untreated congenital hypothyroidism (CH) results in intellectual retardation, there is debate whether mild abnormalities in thyroid function pose any risk to childhood development. Concern over the impact of mildly reduced thyroid function has led to a progressive decrease in CH screening thresholds in many countries. Consequently, the frequency of subclinical congenital hypothyroidism (SCH) has increased dramatically, but it remains unclear whether there are any long-term sequelae. We hypothesised: 1. Neonatal SCH would not persist into mid-childhood. 2. Children with SCH have no adverse cognitive outcomes. Methods: Participants were healthy children aged 6-11 years identified from the National Newborn Screening database as having SCH at birth (n=53), and sibling controls aged 6-16 years (n=50). Screening TSH values of 9-14mIU/L were used. Children underwent cognitive assessments of intelligence, motor function, behaviour, executive functioning, and visual-motor integration. Blood tests were performed on SCH children to determine current thyroid function. Results: TSH levels of SCH children (mean newborn TSH = 10.62 ± 1.43 mIU/L blood) had decreased to within normal range (mean current TSH = 2.31 ± 1.06 mIU/L serum) at the time of cognitive testing. Each 1 mIU/L increase in newborn TSH was associated with decreasing scores of verbal (β=-4.15), non-verbal (β=-2.29), and full-scale (β=-2.89) IQ scores. Overall scores on all cognitive and movement test measures were similar between SCH and sibling control groups. However, SCH females performed more poorly than female controls on behaviour and executive functioning measures (p<.05), whereas SCH males performed better than male controls on movement, behaviour, and executive functioning measures (p<.05). Discussion: TSH levels at birth were associated with reductions in IQ in mid-childhood. We speculate that mild elevations in newborn TSH reflect an environment that predisposes to lower IQ, rather than a direct effect of SCH on neurodevelopment. Nonetheless, SCH was associated with gender-specific neuropsychological effects that are more difficult to explain.