Association between ambient air pollution and thyroid hormones levels: A systematic review and meta-analysis

Growing studies have focused on the effects of ambient air pollution on thyroid hormones (THs), but the results were controversial. Therefore, a systematic review and meta-analysis was conducted by pooling current evidence on this association.

Four databases were searched for studies examining the associations of particulate matter [diameter ≤10 μm (PM₁₀) or ≤2.5 μm (PM_2.5)] and gaseous [sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃), carbon monoxide (CO)] pollutants with THs levels. Random effects models were used to pool the changes in THs levels with increasing air pollutant concentrations. Subgroup analyses were constructed by region, design, sample size, pollutant concentrations, evaluated methods, and potential risk exposure windows.

A total of 14 studies covering 357,226 participants were included in this meta-analysis. The pooled results showed significant associations of exposure to PM_2.5, PM₁₀, NO₂, SO₂, and CO with decreases in free thyroxine (FT4) with percent changes (PC) ranging from -0.593 % to -3.925 %. PM_2.5, NO₂, and CO were negatively associated with levels of FT4/FT3 (PC: from -0.604 % to -2.975 %). In addition, results showed significant associations of PM_2.5 with hypothyroxinemia and high thyroid-stimulating hormone (TSH). Subgroup analyses indicated that PM_2.5 and NO₂ were significantly associated with FT4 in studies of Chinese, and similar significant findings were found in studies of PM_2.5 and FT4/FT3 in areas with higher concentrations of air pollutants and larger samples. PM_2.5 exposure in the first trimester was found to be associated with lower FT4 levels in pregnant women.

Our findings suggest that exposure to air pollution is associated with changes in THs levels. Enhanced management of highly polluted areas, identification of harmful components and sources of PM, and protection from harmful exposures in early pregnancy may be of great public health importance for the population's thyroid function.