Association of Continued Breastfeeding with Micronutrient Status, Thyroid Function, and Cognitive Development in Toddlers

Background : To investigate the associations of continued breastfeeding with micronutrient status, thyroid function, and cognitive development in toddlers. Methods: In this cross-sectional study, 110 toddlers aged 1–2 years were recruited at Shin Kong Wu Ho-Su Memorial Hospital. Participants were categorized into continued breastfeeding (CBF) and discontinued breastfeeding (DCBF) groups. Data collection included anthropometry and Bayley-III cognitive evaluation. Laboratory analyses measured complete blood count, ferritin, 25(OH)D₃, calcium, PTH, TSH, FT3, FT4, and urinary iodine concentration. Results: Toddlers in the CBF group had significantly lower ferritin levels and a higher prevalence of iron deficiency compared with the DCBF group (p < 0.05), while vitamin D insufficiency and suboptimal iodine status were common across both groups. Thyroid hormone profiles of all participants were within normal ranges, but FT4 concentrations were higher in the CBF group ( p  = 0.001) and were positively associated with cognitive, language, and motor scores on the Bayley-III (all p < 0.05). Multivariable regression further confirmed FT4 as a significant predictor of developmental outcomes, whereas male sex predicted lower language performance. Conclusion: Continued breastfeeding was associated with improved cognitive outcomes, potentially mediated by higher FT4 concentrations, but also increased risk of iron deficiency. These findings highlight the need to encourage breastfeeding while ensuring appropriate micronutrient monitoring and supplementation, particularly iron and vitamin D, to optimize toddler development.