Effects of the abdominal fat distribution on the relationship between exposure to air pollutants and thyroid hormones among Korean adult males

BACKGROUND: Several significant associations between air pollution and thyroid function have been reported, but few studies have identified whether these associations differ by obesity, particularly its regional distribution. We assessed the relationship between ambient air pollution and thyroid hormone, and whether this relationship is modified by abdominal adiposity, as indicated by the waist circumference, visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and visceral-to-subcutaneous fat ratio (VSR) in Korean men. METHODS: We included 2440 male adults in the final analysis and used each person’s annual average exposure to four air pollutants: particulate matter with an aerodynamic diameter ≤ 10 μm (PM(10)), nitrogen dioxide, sulfur dioxide (SO(2)), and carbon monoxide (CO). Abdominal fat deposition was quantified by computed tomography. Serum thyrotropin (TSH) and free thyroxine (FT(4)) concentrations were measured for thyroid hormone. To evaluate the relationship between air pollution and thyroid hormone according to adiposity, we performed multiple linear regression analysis on the two subgroups stratified by abdominal fat level. RESULTS: Abdominal adiposity was significantly related to FT(4) concentration. The exposures to air pollutants were associated with increased TSH and decreased FT(4) concentrations. In stratified analysis using abdominal fat traits, ambient air pollution except for SO(2) was significantly related to increased TSH and decreased FT(4) concentrations in the high adiposity group (all p < 0.05), but not in the normal adiposity group. Among the air pollutants, PM(10) showed an association with an increase of TSH concentration in all group with high adiposity, including high VAT, high SAT, and high VSR groups (all p < 0.05). In case of FT(4), CO showed a similar pattern. Among the abdominal fat-related traits, the VSR in the high adiposity group had the largest effect on the relationship between exposure to air pollutants and thyroid hormone. CONCLUSIONS: This study suggests the first clue that the relationship between air pollution exposure and thyroid hormone differs according to abdominal fat distribution among Korean adult males.