Studies on early life ambient air pollution exposures and childhood thyroid function are scarce. This study aimed to evaluate the relationships between early life fine particulate matter (≤2.5 μm; PM_2.5) and nitrogen dioxide (NO₂) exposures and thyroid function in children. We measured the levels of thyrotropin, triiodothyronine, and free thyroxine in children (n = 684) residing in a rural Korean area at age 2, 4, 6, or 8 years from 2012 to 2020 in the Environment and Development of Children cohort. The relationship between residential average exposure levels of PM_2.5 and NO₂ during pregnancy and 1-year average levels before visit and thyroid function during childhood were analyzed. Inverse association between increases of 10 μg/m³ in PM_2.5 during the first trimester and thyrotropin levels at aged 4 (β, -0.12; 95% CI: -0.22, -0.02) and 6 years (β, -0.16; 95% CI: -0.26, -0.06) were observed. No association was found between PM_2.5 exposure during the second and third trimester and childhood TSH levels. Childhood PM_2.5 exposure was positively associated with thyrotropin rise at aged 4 (β, 0.2; 95% CI: 0.06, 0.35) and 6 years (β, 0.16; 95% CI: 0.02, 0.29) and inversely related with free thyroxine levels at aged 8 years (β, -0.04; 95% CI: -0.07, -0.01). No relationship between NO₂ exposure and thyroid function was found. In conclusion, association between PM_2.5 exposure and childhood thyrotropin levels varied depending on exposure timing. Early gestational exposure showed an inverse relationship, whereas childhood exposure were positively associated with childhood thyrotropin levels. The long-term effects of early life air pollution exposure and underlying mechanisms should be investigated in future studies.