• • Median serum ∑8PBDEs was 10.2 ng·g⁻¹ lw (range: 0.13–2089.4 ng·g⁻¹ lw), with BDE-209 contributing 59.7% of total, indicating dominant exposure to this high-molecular-weight congener in the Shenzhen cohort.
• • A 1.7-fold increase in serum BDE-153 was associated with a 0.4% increase in FT3 (P<0.05), suggesting a potential stimulatory effect on thyroid hormone synthesis or release.
• • A 1.7-fold increase in serum BDE-183 was associated with a 0.9% decrease in T3 and a 0.7% decrease in FT3 (P<0.05), indicating a suppressive effect on thyroid hormone levels.
• • BKMR and WQS models revealed that mixed PBDE exposure was negatively associated with TSH at high exposure levels and with T3 and T3/FT3 ratio, with BDE-153 and BDE-183 as key contributors, underscoring the importance of assessing cumulative effects.