Mild Variations in Thyroid Hormone Serum Concentrations Show Associations with the ET 6-6-R Developmental Test:...

Introduction: Thyroid hormones are essential for child development, and untreated hypothyroidism can lead to severe adverse effects, including mental retardation. However, whether thyroid hormone variation within the normal range affects developmental tests is not known. Thus, we aimed to investigate associations between physiological variations in thyroid hormone levels and the results of developmental tests in healthy children.

This study included 946 children who completed the Bayley Scales of Infant and Toddler Development (BSID) III and 1,467 children who completed the developmental test for children from 6 months to 6 years old (ET 6-6-R). Using regression models, we assessed the association between TSH, FT3, and FT4 and test results as outcomes. In addition, we assessed associations between the subscales and stability across age.

Higher TSH-SDS values were associated with lower ET 6-6-R scores in cognitive development, language development, fine and gross motor skills, with effects between β = -0.13 to β = -0.23 (all ps < 0.05). After adjusting for age, sex, and mother's education, only language development reached significance. FT3 was positively associated with gross motor skills (β = 0.17), fine motor skills (β = 0.26), and cognition (β = 0.15) (ps < 0.05). After adjusting, only fine motor skills remained significant. FT4 showed positive but mostly nonsignificant associations with most subscales. We found no consistent significant associations between BSID scores and thyroid hormones.

The differential results for the developmental tests indicate that the associations are highly specific for particular aspects of development and even developmental stages. Therefore, single measurements have very limited explanatory power.