Dose-response relationships between urinary phthalate metabolites and serum thyroid hormones among waste plastic recycling workers in China

Background: Exposure to phthalates may affect thyroid hormone status. However, there were inconsistent observations for the associations of phthalates exposure with altered thyroid hormones.

Objectives: The aim of this study was to investigate effects of urinary excretion of phthalate metabolites on the levels of thyroid hormones among workers engaged in waste plastic recycling in China.

Methods: We measured serum levels of thyroid hormones and urinary levels of eight phthalate metabolites among 317 participants (165 workers engaged in waste plastic recycling and 152 farmers), analyzed relationships between urinary phthalate metabolites and thyroid function parameters by multivariate linear regression analysis and structural equation modelling as well as assessed the dose-response relationships between them by restricted cubic spline functions.

Results: Maximum urinary values of eight phthalate metabolites in the occupational exposed workers were higher than the controls. Compared with the controls, the workers had higher levels of urinary monobenzyl phthalate (MBzP, 1.12 vs. 0.92 μg/g creatinine), mono (2-ethyl-5-hydroxyhexyl) phthalate (MEHHP, 38.84 vs. 32.55 μg/g creatinine), mono-n-octyl phthalate (MOP, .11 vs. 0.09 μg/g creatinine), serum total triiodothyronine (T3, 1.04 vs. 0.92 ng/mL) and the T3 to thyroxine (T4) ratio (1.44 vs. 1.09) (all P < 0.05). The results from structural equation modelling analysis showed that phthalates metabolites were positively associated with total T3 (β = 0.044, SE = 0.021, P < 0.05) or the T3/T4 ratio (β = 0.053, SE = 0.022, P < 0.05) among all participants. Among the workers, there were the non-monotonic dose-response associations between urinary monomethyl phthalate (MMP) and serum total T3 or the T3/T4 ratio, as well as between urinary monoethyl phthalate (MEP) and the T3/T4 ratio (all P < 0.05).

Conclusions: The dose-response relationships between urinary phthalate metabolites and thyroid hormone parameters may be non-monotonic among the workers. Further investigations are needed to corroborate these findings.