Association Between Long-Term Exposure to Environmental Fine Particulate Matter and the Prevalence of Thyroid Disorders: A National Cross-Sectional Study in China

<b><i>Background:</i></b> Exposure to particles with an aerodynamic diameter of ≤2.5 μm (PM_2.5) is associated with the occurrence of thyroid dysfunction among pregnant women and neonates, but it is not known if this association occurs in the general population. We aimed to determine the association of prolonged exposure to PM_2.5 with the prevalence of thyroid disorders among adults in China. <b><i>Methods:</i></b> A nationally representative cross-sectional study of thyroid disorders, iodine status, and diabetes status was carried out in all 31 provinces across China from 2015 to 2017. In total, 73,900 adults aged 18 years and older were included. Serum concentrations of thyroid hormones, thyrotropin, and thyroid antibodies and the urine iodine concentration were measured. The environmental concentration of PM_2.5 for each participant's residential address at a spatial resolution of 1 × 1 km was estimated. <b><i>Results:</i></b> The average long-term exposure to PM_2.5 at residential addresses was 66.41 μg/m³, ranging from 17.58 μg/m³ to 120.40 μg/m³. Compared with that of individuals with lower exposure levels, the prevalence of thyroid diseases such as autoimmune thyroiditis and subclinical hypothyroidism was greater in those with PM_2.5 concentrations within the third quartile range (60.18 to 73.78 μg/m³). Compared with those in the first quartile (17.58 to 46.38 μg/m³), participants in the highest PM_2.5 quartile (73.78 to 120.40 μg/m³) presented an increased risk of overt hypothyroidism (OR 1.23 [CI 0.94-1.61]), subclinical hypothyroidism (1.10 [1.01-1.21]), autoimmune thyroiditis (1.09 [1.00-1.18]), and thyroglobulin antibody positivity (1.17 [1.07-1.29]). However, there was no association between PM_2.5 exposure and overt hyperthyroidism, subclinical hyperthyroidism, Graves' disease, or thyroid peroxidase antibody positivity (<i>p</i> > 0.05). Each 10 μg/m³ increase in the PM_2.5 concentration was associated with an increased risk of overt hypothyroidism (OR 1.05 [1.00-1.11]), subclinical hypothyroidism (1.02 [1.00-1.03]), and thyroglobulin antibody positivity (1.02 [1.00-1.04]). Furthermore, a nearly linear exposure-response relationship was observed between long-term PM_2.5 exposure and thyroglobulin antibody positivity. <b><i>Conclusions:</i></b> PM_2.5 exposure was associated with thyroid disorders among Chinese adults. A dose-response relationship between PM_2.5 exposure and autoimmune thyroiditis, as well as thyroglobulin antibody positivity, was also observed.