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Objective This study aimed to evaluate the association between occupational radiation exposure and changes in thyroid hormone levels among medical radiation workers. Methods This retrospective cohort study included 2,946 radiation workers from 20 Guangzhou hospitals. Data on general characteristics, participant radiation dosimetry, and thyroid function test results [thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroid hormone (T4)] were extracted from dosimetry and medical records. The generalized estimating equation was used to evaluate the trend of changes in thyroid hormone levels over time and was adjusted for age, gender, and occupation. Results The average annual effective dose was very low and showed a general downward trend. During the follow-up period, changes in T3 and T4 levels among radiation workers were –0.015 [95％confidence interval (CI) –0.018 to –0.012] nmol/L per year and –2.294 (95％ CI –2.426 to –2.162) nmol/L per year, respectively. Thyroid hormone levels were significantly different between males and females. T3 levels in the group of upper quartile of dose were significantly higher than in the lower quartile group (P = 0.006). No significant decreased trend in thyroid hormone levels was observed with increasing average effective doses. Conclusion Thyroid hormone secretion might be affected even in low-dose radiation exposure environments.