Abstract： Objective　To analyze heart rate variability (HRV)and its changes with thyroxine treatment in patients with hypothyroidism. Methods　HRV was analyzed using 24-hour electrocardiographic recording in 38 patients with hypothyroidism and 21 normal controls. The changes in HRV were evaluated for the 18 hypothyroid patients after 3 months of thyroxine therapy.Results　The time domain measurements of HRV in hypothyroid patients were much lower than those in the control group. As to HRV frequency domain, the high frequency power was significantly higher, but the ratio of low frequency power to frequency power for hypothyroid patients was lower than in the controls. These abnormal changes of HRV measurements in hypothyroid patients were improved after treatment with thyroxine and were associated with recovery of serum concentrations of FT3 and FT4. Conclusions　Patients with hypothyroidism often have autonomic neuropathies with a higher level of vagal tone. These abnormalities could be partly improved by thyoxine therapy.