Patients with hyperthyroidism typically have both increased sympathetic and decreased vagal modulation of heart rate, Taiwanese study findings show.
"The clinical manifestations of hyperthyroidism resemble those of the hyperadrenergic state," the researchers, led by Woei-Chyn Chu, from the National Yang-Ming University in Taipei, note.
To investigate the relationship between hyperthyroidism and cardiac autonomic nervous function, the team compared heart rate variability (HRV) in 32 hyperthyroid Graves' disease patients and 32 age-, gender-, and body mass index-matched control individuals. The patients and controls were aged an average of 31 years.
Spectral analysis revealed significant differences in HRV between the patients and the controls.
Specifically, there were decreases in total power, very low frequency power, low frequency (LF) power, and high frequency (HF) power, in hyperthyroid individuals compared with non-hyperthyroid individuals.
Moreover, participants with hyperthyroidism showed a reduction in HF power in normalized units (HF%), and an elevation in LF power in normalized units (LF%), and in the ratio of LF to HF power (LF/HF).
Serum thyroid hormone concentrations were significantly correlated with spectral HRV parameters in all the patients.
Importantly, these HRV parameters normalized to those of the controls after corrective hyperthyroidism therapy in 28 of the patients, the investigators report in an early advance publication by the journal Clinical Endocrinology.
"Hyperthyroidism is a sympathovagal imbalanced state, characterized by both increased sympathetic and decreased vagal modulation of the heart rate," Chu et al summarize.
"These autonomic dysfunctions can be detected simultaneously by spectral analysis of HRV, and the spectral HRV parameters could reflect the disease severity in hyperthyroid patients," they suggest.