We maintained thyroidectomized Wistar male rats for 70 weeks to evaluate the electrocardiographic e#ects of long-term hypothyroidism, which occurs in humans under treatment with amiodarone, a class III antiarrhythmic drug. We monitored the electrocardiographic changes in the hypothyroid rats up to 70 weeks after the thyroidectomy. Plasma levels of thyroid hormones significantly decreased starting 1 week after the thyroidectomy and remained low until the end of the 70-week observation period.

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... longation of the QT interval and bradycardia were significant from 1 week. However, the QT prolongation rate at 1 and 70 weeks showed no di#erence, indicating that QT prolongation was not enhanced by the hypothyroid state over the long term. In addition, there were significant correlations between plasma thyroid hormone levels and QT intervals or heart rates of the thyroidectomized animals. This finding suggests that these electrocardiographic changes were caused by hypothyroidism. Changes in the QT interval of animals that subsequently died or were sacrificed and in that of surviving ones showed no di#erence, and there was no arrhythmia detected during any of the recordings. Therefore, it is conceivable that the cause of the critical state or death might not be connected with the QT prolongation-related abnormal electrocardiographic changes.