Shift work, thyroid function and liver function among subway workers

Shift work is associated with an increased risk of liver injury. However, whether and how shift work alters liver function remains unclear. This study aimed to investigate the associations between shift work and the liver function parameters, and further explore the mediating roles of thyroid function indicators. We conducted a cross-sectional study involving a convenience sample of 724 subway workers. Multivariate linear regression models were adopted to approximate the effect values for the associations of shift work with liver function parameters and thyroid function indicators. Mediation analyses were used to explore the roles of thyroid function indicators in the association between shift work and liver function parameters. Shift work was associated with increased levels of alkaline phosphatase (ALP), total triiodothyronine (TT3) and free thyroxine (FT4) (β = 6.309, 95% confidence interval [CI] 2.739-9.879, β = 0.328, 95% CI 0.242-0.415 and β = 2.913, 95% CI 1.502-2.884, respectively). In stratification analysis, the positive association between shift work and TT3, FT3 and FT4 was more pronounced among people >30 years old. The increase in shift worker FT3 and aspartate transaminase levels was stronger among alcohol users. Mediation analysis showed that TT3 and FT4 mediated 39% and 29% of the associations between shift work and the increased level of ALP, respectively (all P < 0.05). The results suggest that shift work is associated with increased ALP levels of subway workers, which is partly mediated by the increase of TT3 and FT4 levels.