Abstract
The aim of this study is to document the impact of thyroid hormones on the autonomic nervous system (ANS), expressed by heart rate variability (HRV) and heart rate turbulence (HRT), in patients with hyperthyroidism, compared to controls. Another purpose is to determine if HRV and HRT parameters are significantly correlated with the levels of thyroid hormones and the duration of hyperthyroidism. We studied HRV, in time and frequency domain, and HRT in a group of 113 premenopausal women, with still untreated overt hyperthyroidism, without cardiovascular diseases or risk factors for atherosclerosis, admitted between 2015 and 2018 in the Endocrinology Clinic of our hospital and compared the results with data obtained in 29 healthy controls. Depending on the severity and duration of hyperthyroidism and levels of thyroid hormones, patients were assigned to three groups: mild and moderate hyperthyroidism, severe and recurrent forms. We performed 24 hours Holter monitoring in patients and controls. Regarding HRV parameters in time domain, they were significantly depressed in patients comparing to controls. Referring to HRT, all patients had abnormal, positive values of turbulence onset (TO) and we documented statistically significant differences (p[0.001) when compared to controls. All patients had normal positive values of turbulence slope (TS), which decreased parallel with the severity of hyperthyroidism. In patients with hyperthyroidism, we documented depressed values of HRV parameters in time domain, correlated with the duration and severity of the thyroid disease. Our patients had pathologic values of TO. Although positive, TS values were lower when compared to controls.
Highlights
Hyperthyroidism determines a hyperkinetic state suggesting an imbalance of the autonomic nervous system (ANS) [1]
Because in patients with hyperthyroidism, normal or even lower levels of epinephrine and norepinephrine have been determined in serum and urine, it was assumed that the myocardium presents an increased sensitivity to catecholamines, possibly associated with an increased β-receptor density
Regarding heart rate variability (HRV), we studied the following parameters in time domain (TD): the standard deviation of all normal to normal (NN) intervals (SDNN), the standard deviation of all NN intervals occurred in 5 minutes (SDANN), the radical of the differences of mean squared
Summary
Hyperthyroidism determines a hyperkinetic state suggesting an imbalance of the autonomic nervous system (ANS) [1] These alterations of the ANS balance can be documented by modifications of heart rate variability (HRV). HRT studies the sinus rhythm cycle length variation after isolated premature ventricular contractions (PVC) These methods are used in studies to estimate the presence of sympathovagal imbalance in patients with congestive heart failure [2], or after myocardial infarction [5], in order to predict an increased cardiovascular risk and morbidity [5]. The significance of these methods for the assessment of the cardiovascular risk in hyperthyroid patients, without other structural heart diseases, except the ones induced by thyroid hormones in excess, is still a subject to debate [8,9]. In the heart, thyroid hormones increase the automatism and the intrinsic activity of the sinus node and reduce the vagal tone [1113]
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