title: Changes in heart rate variability and QT dispersion in patients with overt hypothyroidism.
abstract: OBJECTIVE
  The aim of the present study was to evaluate the effect of clinical hypothyroidism on cardiovascular autonomic function and ventricular repolarization.
  
  
  METHODS
  We studied 31 patients (22 females and 9 males; mean age 53.6+/-11.8 years) with overt hypothyroidism (TSH=56.2+/-14.7 microU/ml, low free thyroxine (T4), free tri-iodothyronine (T3)) and 31 euthyroid controls, to investigate the dispersion of the QT interval in electrocardiogram (ECG) (an index of inhomogeneity of repolarization) and heart rate variability (HRV; a measure of cardiac autonomic modulation). The hypothyroid patients and controls underwent a full medical examination, standard 12-lead ECG, and 24-h ambulatory ECG monitoring. The hypothyroid patients were re-examined after 6 months of treatment with L-T4.
  
  
  RESULTS
  Patients with hypothyroidism showed higher QT dispersion and lower HRV measures than controls (P<0.01 or P<0.001). In hypothyroid patients, standard deviation of all R-R intervals was inversely related (by simple regression) to serum (log)TSH levels (r=-0.47, P=0.008), while QT dispersion (r=0.50, P=0.004) and QTc dispersion (r=0.46, P=0.008) were directly related to (log)TSH. Parameters of HRV improved after 6 months of L-T4 treatment, with the correction of hypothyroidism, becoming comparable with those of the control subjects, whereas the QT and QTc dispersion results were found to be only partially restored, remaining higher than the controls.
  
  
  CONCLUSIONS
  The results of the study demonstrate that hypothyroidism is associated with a decreased sympatho-vagal modulation of the heart rate and with an increased inhomogeneity of ventricular recovery times. The assessment of HRV and QT dispersion in patients with overt hypothyroidism may represent a useful tool in monitoring the cardiovascular risks.
authors.affiliation: Department of Internal Medicine, School of Medicine, University of Pisa, Via Roma, 67, I-56100 Pisa, Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Galetta
authors.forename: Ferdinando
authors.initials: F
authors.lastname: Franzoni
authors.forename: Poupak
authors.initials: P
authors.lastname: Fallahi
authors.forename: Leonardo
authors.initials: L
authors.lastname: Tocchini
authors.forename: Lara
authors.initials: L
authors.lastname: Braccini
authors.forename: Gino
authors.initials: G
authors.lastname: Santoro
authors.forename: Alessandro
authors.initials: A
authors.lastname: Antonelli
mesh_terms: D000328:Adult; D002940:Circadian Rhythm / Q000502:physiology; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D007037:Hypothyroidism / Q000097:blood / Q000473:pathology / Q000503:physiopathology*; D008133:Long QT Syndrome / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine