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The observation that hyperthyroid patients manifest symptoms and signs similar to those of hyperadrenergic states suggests autonomic dysfunctions in hyperthyroidism. The analysis of heart rate variability (HRV) has been used to understand the modulation of the cardiac autonomic nervous system and the characteristics of the heart rate dynamics. Time and frequency domain HRV parameters, along with correlation dimension (CD) and detrended fluctuation analysis (DFA), were assessed from 30 min electrocardiogram recordings in 25 newly diagnosed hyperthyroid Graves’ disease patients and 25 sex-, age-, and body mass index-matched normal controls. Compared to the normal controls, the hyperthyroid patients revealed significant differences in the following HRV parameters: a decrease in the standard deviation of RR intervals, total power, high frequency power (HF) and CD; and an increase in low frequency power (LF) in normalized units, the ratio of LF to HF, and DFA. We conclude that hyperthyroidism is characterized by both increased sympathetic and decreased vagal modulation of the heart rate. In addition, the fractal analysis of HRV indicates reduced complexity and impaired tolerance to cardiovascular stresses in hyperthyroidism.
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