Amiodarone is an effective antiarrhythmic drug. However, it is associated with changes in thyroid function in euthyroid patients due to its high iodine content and intrinsic drug effects. Studies have been conducted in iodine-deficient and iodine-sufficient countries; however, data from countries with excessive iodine intake are lacking. Thus, this study aimed to evaluate the effect of long-term amiodarone treatment on thyroid function in euthyroid Japanese patients. Japanese adults aged ≥18 years who were treated with amiodarone for at least 90 consecutive days were included in this retrospective chart review. Patients with abnormal thyroid function test results at baseline were excluded. Serial changes in thyroid function tests at baseline and at days 30, 90, 180, 270, and 360 were analyzed using a mixed-effects model for repeated measures. In total, 46 patients with a mean age of 63.7 years were evaluated. The mean TSH level significantly increased from 1.62 μIU/mL at baseline to 3.43, 2.75, 2.84, 2.78, and 2.65 μIU/mL at days 30, 90, 180, 270, and 360, respectively. The mean free T4 level significantly increased from 1.3 ng/dL at baseline to 1.4, 1.5, 1.5, 1.5, and 1.5 ng/dL at days 30, 90, 180, 270, and 360, respectively. The mean free T3 level significantly decreased from 2.8 pg/mL at baseline to 2.4, 2.3, 2.3, 2.4, and 2.4 pg/mL at days 30, 90, 180, 270, and 360, respectively. In conclusion, significant changes in thyroid function persisted not only in the acute phase but also in the chronic phase of long-term amiodarone treatment.
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