Although a significant relationship has been reported between air pollution and thyroid function in limited samples or regions, few studies have addressed this association in the general population.Using a nationwide sample of Korean adults, we investigated the association between exposure to air pollution and thyroid function, and whether this association differed between subgroups stratified according to age or body mass index (BMI).We included 4,704 adults in the final analysis and used each person's annual average exposure to four air pollutants, namely, particulate matter with an aerodynamic diameter ≤10 μm (PM10), nitrogen dioxide (NO2), sulfur dioxide, and carbon monoxide (CO). We measured serum thyroid-stimulating hormone (TSH) and free thyroxine (FT4) concentrations as indicators of thyroid function according to age and BMI.The annual average exposure to NO2 and CO was significantly associated with an elevated TSH and reduced FT4 concentration after adjusting for possible confounding factors (all p < 0.05). In men, in addition to these two pollutants, PM10 exposure was positively associated with TSH level (p = 0.03). Age-stratified analysis showed stronger effects of NO2 and CO exposure in older than in younger adults. Exposure to these air pollutants was related to serum TSH and FT4 concentrations in people with overweight or obesity but not in those of normal weight.This study provides the first evidence that air pollution exposure is linked to thyroid function in the general population and that this association may be stronger in older or overweight or obese adults.