OBJECTIVE: Investigate the association between the age of treatment onset and confirmatory TSH level (as an indicator of severity) with a greater risk of developmental delay in infants with congenital hypothyroidism (CH). METHOD: The authors conducted a cross-sectional, observational, unmatched case-control study at a Brazilian neonatal screening reference center. Seventy-seven infants with CH (mean age: 12 ± 6.4 months) were examined. The authors evaluated their performance using the Bayley-III Screening Test and categorized them as "LOWER RISK" (competent category) or "GREATER RISK" (combined at-risk + emergent categories) for developmental delay based on the 25 RESULTS: Infants with CH are at a higher risk of non-competent performance in cognition, receptive language, fine motor skills, and gross motor skills when compared to infants without CH. This risk is more pronounced in infants with more severe indications of CH (TSH >
30 μUI/L in the confirmatory test) for cognition (OR = 5.64
p = 0.01), receptive language (OR = 14.68
p = 0.000), fine motor skills (OR = 8.25
p = 0.000), and gross motor skills (OR = 5.00
p = 0.011). CONCLUSION: The level of TSH in the confirmatory test can be a good indicator for identifying infants with CH who are at a higher risk of non-competent performance in cognition, receptive language, and motor skills. Monitoring development, early detection of delays, and intervention programs are particularly important for infants with CH.