The effects of ionized radiation on the thyroid have been extensively studied. However, most studies have focused on high-dose radiation received accidentally or through therapy, and few were on low-dose occupational exposure.Using a retrospective cohort study design, we collected health examination reports from employees who worked on jobs with occupational exposure to radiation at a hospital to evaluate possible changes in the serum thyroid hormones and determine whether there is a dose-response effect. After excluding those with diseases that may affect thyroid function and who were pregnant at any given examination during the study periods we followed the remaining 326 workers for 12 years and evaluated the associations between radiation exposure and changes in serum thyroid hormones using the generalized estimating equation for repeated measures. Data from an external comparison cohort were used to adjust for changes over time.We observed declines in triiodothyronine (T3) and thyroxine (T4) over the study period, but not in thyroid-stimulating hormone (TSH). In addition, we found negative dose-response relationships between exposure duration and declines in the serum levels of T3 (a change of -0.037 ng/ml/year after adjusting for sex and age at the beginning of follow-up; 95% confidence interval [CI] = -0.042, -0.032 ng/ml/year) and T4 (-0.115 µg/dl/year; 95% CI = -0.140, -0.091 µg/dl/year). We also observed an increase in the TSH level (0.683 µIU/ml/year; 95% CI = 0.151, 1.214 µIU/ml/year) after the ninth year of follow-up.We concluded that despite low exposure doses, occupational exposure to ionizing radiation in healthcare workers still may be associated with the declines in the serum levels of T3 and T4.