Background: Thyroid hormones exert effects on the heart and peripheral circulation. Heart rate variability (HRV) is a good marker for identifying the cardiovascular risk in patients with hyperthyroidism. Aims and Objective: To evaluate the impact of hyperthyroidism on autonomic tone and to compare the HRV parameters in patients with hyperthyroidism and healthy volunteers. Materials and Methods: This study was conducted at PSG Institute of Medical Sciences and Research, Coimbatore, Tamil Nadu, on 30 hyperthyroid patients (cases) with average age 39.23 ± 6.91 years and 30 healthy volunteers (controls) with average age 42.13 ± 6.78 years. After obtaining the written consent from the subjects, ECG was recorded for 10 min in a computerized physiograph in lead II. Results: Statistical analysis carried out by independent Student's t-test showed a significant difference (P < 0.001) in HRV parameters of patients with hyperthyroid and healthy volunteers. Among the frequency domain measures, a decrease in high frequency (HF) in normalized units and increase in low frequency (LF) in normalized units, and in the ratio of LF to HF (LF/HF) in hyperthyroid patients was observed. Among the time domain measures, decrease in RR intervals, SDNN, RMSSD, and NN50 was observed in patients with hyperthyroid compared to those with euthyroidism. Conclusion: This study showed that individuals with hyperthyroidism had a higher sympathetic tone and a lower parasympathetic tone as compared to those with euthyroidism. Using HRV analysis, we can identify patients who are at risk for cardiac complications and early intervention can reduce mortality rates.

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