Power spectral analysis of variations in heart rate in patients with hyperthyroidism or hypothyroidism

Power spectral analysis (PSA) of the variation in heart rate is useful in determining the relative activity of the sympathetic and parasympathetic nerves. In this study, PSA was used to investigate the relationship between abnormalities in autonomic nerve function and the presence of thyroid disorders in patients with autoimmune thyroid diseases. The low frequency (LF) or high frequency (HF) components of R-R variations were determined by PSA. The coefficient of variation of the R-R time intervals (CV(R-R)) was positively correlated with HF in healthy subjects. In untreated hyperthyroid patients with Graves' disease, the CV(R-R) and HF values were significantly lower than in healthy controls. Moreover, the LF/HF ratio in patients with untreated Graves' disease was significantly higher, and the LF/HF ratio in hypothyroid patients with Hashimoto's thyroiditis was significantly lower than in healthy controls. A negative correlation was observed between serum levels of free thyroid hormones (FT4 and FT3) and HF in Graves' disease patients. In some hyperthyroid patients, antithyroid drug therapy or beta-blocker administration gradually restored reduced HF values. Present results suggest that relative vagal nerve activity is reduced in hyperthyroid patients and that this reduction is reversible according to the decrease in serum levels of thyroid hormones.