Continuous 24-hour electrocardiography in thyrotoxicosis before and after treatment

Ten thyrotoxic individuals, who otherwise had no evidence of cardiovascular disease, underwent continuous ambulatory 24-hour ECG monitoring, before and after antithyroid treatment. The mean age of the subjects was 41 +/- 14.4 years (mean +/- SD) with a range of 22 to 66 years. When subjects were thyrotoxic, the mean heart rate for the group was 104 +/- 10.8 bpm. This fell to 82 +/- 6.8 bpm when the subjects were rendered euthyroid (p less than 0.001). Circadian rhythm of heart rate response was maintained in the thyrotoxic state, although heart rate variability was significantly increased (p less than 0.001). The prevalence of ventricular premature contractions was not significantly different before and after treatment, although premature atrial contractions were more prevalent during the middle third of the day (p less than 0.01) when subjects were euthyroid. These findings support the view that normal adrenergic responsiveness persists in hyperthyroidism, and for most individuals treatment does not significantly alter the prevalence of ectopic activity.