Long-term ECG recordings in hyperthyroid patients before and after antithyroid treatment

Heart rate and heart rhythm were studied in 19 hyperthyroid patients before and after antithyroid treatment inducing a euthyroid state. The mean 24-hour heart rate in patients with sinus rhythm and without drugs influencing heart rate was 100 beats/min before and 80 after antithyroid treatment. The difference was greatest in the sleeping hours. The heart rate, especially in the sleeping hours, correlated significantly with serum triiodothyronine but not with serum thyroxine concentrations.