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 parasympa thetic nerves. In this study, PSA was used to investigate the relation ship between abnormalities in autonomic nerve function and the presenc e of thyroid disorders in patients with autoimmune thyroid diseases. T he low frequency (LF) or high frequency (HF) components of R-R variati ons were determined by PSA. The coefficient of variation of the R-R ti me intervals (CVR-R) was positively correlated with HF in healthy subj ects. In untreated hyperthyroid patients with Graves' disease, the CVR -R and HF values were significantly lower than in healthy controls. Mo reover, the LF/HF ratio in patients with untreated Graves' disease was significantly higher, and the LF/HF ratio in hypothyroid patients wit h Hashimoto's thyroiditis was significantly lower than in healthy cont rols. 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 s uggest that relative vagal nerve activity is reduced in hyperthyroid p atients and that this reduction is reversible according to the decreas e in serum levels of thyroid hormones.