Objective: To assess autonomic nervous system (ANS) function in patients with hyperthyroidism by the heart rate variability (HRV) and to analyse the impact of hyperthyroidism on the cardiovascular system. Methods: Indexes of HRV were measured in 13 hyperthyroidism patients and 26 healthy controls. Time domain indexes and spectral indexes of HRV were derived. Results : 1. There was marked difference in both HFNU and LF/HF between hyperthyroidism and the control group over a 24-hour period. 2. In patients in control group

there were significant increases in the indexes of LFNU and LF/HF ratio during waking hours

whereas both HFNU and HF bands significantly increased during sleeping hours. In patients with hyperthyroidism

there were no significant differences in all spectral indexes of HRV. 3. There were significant differences in patients with hyperthyroidism in any time domain indexes of HRV compared with control group over a 24-hours period. In hyperthyroidism group

the night-day difference in HRV was significantly smaller compared with the control group. Conclusion: In patients with hyperthyroidism an autonomic imbalance and thyroid hormone excess may determine reduced parasympathetic activity and

thus

leading to a relative hypersympathetic tone. It will be important to adjust the abnormal cardiac autonomic activity promptly.