Developmental Outcomes in Children Aged 3–5.5 Years with Congenital Hypothyroidism: A Case–Control Study Based on the Ages and Stages Questionnaire

Background This study aimed to assess motor, cognitive, and language development in children with congenital hypothyroidism (CH) and compare them with their healthy controls at 36 and 60 months of age. Design: Cross-sectional, observational case–control study Methods This study was conducted in Qom Province (2021–2022) and included 130 children with CH and 130 age-matched healthy controls. Developmental assessments were performed using the standard Ages and Stages Questionnaire – 3rd edition questionnaire (ASQ-3). Independent t-tests and analysis of variance were applied using SPSS version 24. Results At 36 months, 43.3% of children with congenital hypothyroidism showed abnormal scores in at least one developmental domain, compared with 10% of controls (p < 0.01). The most significant differences were observed in fine motor skills (36 months: 49.52 ± 13.31 vs. 56.15 ± 6.46; 60 months: 46.70 ± 16.19 vs. 54.46 ± 6.77; p < 0.001) and in problem-solving at 60 months (46.70 ± 12.39 vs. 54.57 ± 7.29; p < 0.001). Age at treatment initiation showed a significant inverse correlation with communication, gross motor, fine motor, and problem-solving scores, whereas initial T4 and TSH levels were not associated with developmental outcomes. Developmental scores did not differ significantly between undertreatment, overtreatment, and adequately treated subgroups. Conclusion Developmental differences were more pronounced at 60 months than at 36 months, highlighting the potential cumulative impact of congenital hypothyroidism. Early diagnosis and treatment may limit developmental impairments, particularly in most domains. However, fine motor and problem-solving skills appeared more resistant to treatment, emphasizing the need for ongoing developmental monitoring in children with congenital hypothyroidism.