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Heart rate variability and QT dispersion in patients with subclinical hypothyroidism . UNLABELLED The effect of subclinical hypothyroidism ( SH ) on cardiovascular autonomic function and ventricular repolarization has not been yet elucidated . The aim of the present study was to evaluate the dispersion of QT interval , i.e . an index of inhomogeneity of repolarization , and heart rate variability ( HRV ) , i.e . a measure of cardiac autonomic modulation , in SH patients . METHODS The study included 42 patients ( 29 women and 13 men ; mean age 53.2+/-14.2 years ; body surface area 1.76+/-0.14 m2 ) with SH , as judged by elevated serum TSH levels ( > 3.6 mIU/l ; range , 3.8-12.0 ) and normal free thyroid hormones ( FT4 and FT3 ) and 30 euthyroid volunteer . Subjects with cardiac , metabolic , neurological disease or any other systemic disease that could affect autonomic activity were excluded from the study . Patients with SH and control subjects underwent a full history , physical examination , standard 12-lead ECG , and 24-h ambulatory ECG monitoring . To evaluate the effect of treatment with L-thyroxine on QT dispersion and HRV , 15 patients with SH were randomly assigned to receive therapy with L-thyroxine . All the subjects were evaluated at enrolment and after 6 months . RESULTS Patients with SH showed higher QT dispersion and lower HRV measures than healthy controls ( P < 0.01 for all ) . In SH patients , the standard deviation of N-Ns ( SDNN ) was negatively related to TSH ( r=-0.42 , P=0.006 ) , while low frequency ( LF ) /high frequency ( HF ) ratio was positively related to TSH ( r=0.42 , P=0.006 ) . Moreover , in SH patients both QT dispersion and QTc dispersion were positively related to TSH ( r=0.64 and r=0.63 , P < 0.001 for both ) . After 6 months , the patients treated with L-tiroxine exhibited a reduction of QT dispersion and an increase of HRV parameters . CONCLUSION The results of the present study demonstrated that SH can alter autonomic modulation of heart rate and cause increased inhomogeneity of ventricular recovery times . Accordingly , early L-thyroxine treatment may be advised not only to prevent progression to overt hypothyroidism but also to improve abnormal cardiac autonomic function and ventricular repolarization inhomogeneity .
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Intervention_Physical
12-lead ECG , and 24-h ambulatory ECG monitoring
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Intervention_Pharmacological
L-thyroxine
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Intervention_Pharmacological
L-thyroxine
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Intervention_Pharmacological
L-tiroxine
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Intervention_Pharmacological
L-thyroxine
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Outcome_Physical
Heart rate variability
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Outcome_Physical
QT dispersion
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Outcome_Physical
QT dispersion
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Outcome_Physical
lower HRV measures
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Outcome_Physical
standard deviation of N-Ns ( SDNN )
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Outcome_Physical
low frequency ( LF ) /high frequency ( HF ) ratio
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Outcome_Physical
QT dispersion
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Outcome_Physical
QTc dispersion
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Outcome_Physical
QT dispersion
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Outcome_Physical
HRV parameters
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Outcome_Physical
heart rate
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Participant_Condition
subclinical hypothyroidism
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Participant_Condition
subclinical hypothyroidism
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Participant_Condition
SH
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Participant_Sample-size
42 patients
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Participant_Sample-size
29
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Participant_Age
women
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Participant_Sample-size
13
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Participant_Age
men
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Participant_Age
53.2+/-14.2
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Participant_Condition
SH
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Participant_Condition
cardiac , metabolic , neurological disease
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Participant_Condition
systemic disease
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Participant_Condition
SH
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Participant_Condition
SH
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Participant_Condition
SH