hypothyroidism has various cardiovascular manifestations including impaired diastolic function , reduced contractility and infrequently pericardial effusion and heart failure ( 1 - 3 ) .
electrocardiographic ( ecg ) changes in hypothyroidism were bradycardia , rbbb , flat or inverted t wave , qrs prolongation , qt prolongation and infrequently ventricular arrhythmia , torsades de pointes ( 1 - 3 ) .
the qt dispersion is the interlead variability of qt interval on surface ecg that reflects regional variations in myocardial repolarization .
increased the qt dispersion has been linked to the occurrence of malignant ventricular arrhythmias and sudden cardiac death ( 4 - 6 ) .
however , it was reported that the deficiency of thyroid hormone did not affect ecg findings of congenitally hypothyroid neonates ( 7 ) . therefore , the effects of l - thyroxine treatment on ecg parameters , such as qt dispersion , in patients with primary hypothyroidism were examined .
this retrospective study involved 18 patients ( 3 men , 15 women , age : 4818 yr ) with primary hypothyroidism .
the patient taking the qt - prolonging drugs or who had histories of cardiac disease were excluded .
all patient were checked with serum free t4 , thyroid - stimulating hormone ( tsh ) and standard 12-lead ecgs before and after l - thyroxine treatment . after a rest period ,
the electrocardiogram had been checked with 12-lead by electrocardiography recorder ( 25 mm / sec , marquette , fairfield , connecticut , u.s.a . ) .
all measurements were obtained manually by 1 observer unaware of patient identity , diagnosis , and therapy .
the qt interval was measured as the distance between onset of the qrs complex and the end of the t wave in all 12-lead . when the u wave interrupted , the end of the t wave was estimated by extrapolation of the descending t - wave lined down to the baseline .
the qtd was defined as the difference between the maximum qt and the minimum qt across all 12 leads .
the data were analyzed using a statistical package ( spss window 10.0 ver ) . whenever the data could be paired , these were analyzed using paired t - tests .
the thyroid function test , before and after l - thyroixine treatment , is shown in table 1 .
the qt prolongation was detected in 6 patients ( 33% ) at the study baseline .
the t - inversion appeared in 3 patients , and after the l - thyroxine treatment , all patients were normalized ( fig .
the qtc prolongation correlated positively with the baseline tsh level ( r=0.484 , p=0.042 ) .
it was found that after the l - thyroxine treatment , the qt interval ( p<0.05 ) , qtc interval ( p<0.05 ) , qt dispersion ( p=0.008 ) , and the qtc dispersion ( p=0.005 ) significantly decreased .
there were no significant changes in the pr interval , rr interval , nor the qrs duration ( table 2 ) .
the thyroid function test , before and after l - thyroixine treatment , is shown in table 1 .
the qt prolongation was detected in 6 patients ( 33% ) at the study baseline .
the t - inversion appeared in 3 patients , and after the l - thyroxine treatment , all patients were normalized ( fig .
the qtc prolongation correlated positively with the baseline tsh level ( r=0.484 , p=0.042 ) .
it was found that after the l - thyroxine treatment , the qt interval ( p<0.05 ) , qtc interval ( p<0.05 ) , qt dispersion ( p=0.008 ) , and the qtc dispersion ( p=0.005 ) significantly decreased .
there were no significant changes in the pr interval , rr interval , nor the qrs duration ( table 2 ) .
it is well documented that hypothyroidism affects cardiac functions and have various cardiovascular manifestations including impaired diastolic functions , reduced contractility and infrequently pericardial effusion and heart failure .
hypothyroidism classically has been associated with bradycardia , but the degree of slowing heart rate is often modest .
other electrocardiographic changes in hypothyroidism were rbbb , flat or inverted t waves , qrs prolongation , qt prolongation and infrequently ventricular arrhythmia , torsades de pointes ( 1 - 3 ) .
the function of the atrial pacemaker is normal and atrial ectopy is rare , but premature ventricular beats and occasionally ventricular tachycardia can occur ( 8) .
this is in contrasts to thyrotoxicosis , in which atrial tachyarrhythmias are common and ventricular arrhythmias are rare .
the syndrome of torsade de pointes with a long qt interval and ventricular tachycardia can occur with hypothyroidism and be resolved with t4 treatment alone ( 3 , 9 ) .
the duration of the action potential may be prolonged , perhaps reflecting a decrease in voltage - gated potassium channels .
nathaniel et al . reported that significant prolongation of the qtc interval occurred in inadequately treated hypothyroidism and the degree of the qtc prolongation was directly related to the severity of hypothyroidism ( 2 ) .
showed that qt prologation and increased qtd were directly related to the tsh levels in hypothyroidism ( 10 ) .
increased qtd has been found to be associated with an increased incidence of malignant ventricular arrhythmias and sudden death ( 4 - 6 ) .
the qtd provides a potentially simple , inexpensive , noninvasive method of measuring underlying dispersion recovery of ventricular excitability , and should be defined in a way that most accurately reflects this state ( 11 ) .
clinical observations showed that ventricular arrhythmias and sudden death are uncommon in hypothyroidism , despite the marked lengthening of the qt interval ( 3 , 12 , 13 ) .
however , increased qtd in hypothyroidism may facilitate ventricular arrhythmias with hypokalemia , hypomagnesemia , long qt syndrome or drugs .
abnormal changes of heart rate variability measurements and its improvement after treatment in hypothyroidism were reported ( 14 ) .
however , the deficiency of the thyroid hormone did not affect ecg findings of congenitally hypothyroid neonate ( 7 ) .
it was necessary to clarify the effects of l - thyroxine treatment on ecg parameters in primary hypothyroidism . in this study
, our results showed that the qtc and qt dispersions improved after the l - thyroxine treatment in patients with primary hypothyroidism .
this suggests that the thyroid hormone affects ventricular inhomogenicity , and subsequent l - thyroxine replacement therapy may reduce malignant ventricular arrhythmia and sudden cardiac death in primary hypothyroidism .
 hypothyroidism has various cardiovascular manifestation and exhibits electrocardiographic change . 
 the qt dispersion on surface ecg reflects regional variations in myocardial repolarization . 
 the effect of l - thyroxine treatment on ecg parameters , such as qt dispersion , in patients with primary hypothyroidism were investigated . 
 this study involved 18 patients ( 3 men , 15 women , ages : 4818 yr ) with primary hypothyroidism . 
 all patients were checked with a standard 12-lead ecg before and after l - thyroxine treatment . 
 various ecg parameters were then measured twice . 
 the mean l - thyroxine treatment duration was 222.7 months . 
 the mean thyroid - stimulating hormone levels of patients before and after therapy were 40.229.8 u / ml , 3.64.6 u / ml ( p<0.001 ) and free - t4 levels were 0.440.38 ng / dl , 1.510.39 ng / dl ( p<0.001 ) . 
 after l - thyroxine treatment , qt interval ( 39542 vs. 38024 msec , p<0.05 ) , qtc interval ( 43432 vs. 41723 msec , p<0.05 ) , qt dispersion ( 4523 vs. 3013 msec , p=0.008 ) , qtc dispersion ( 4923 vs. 3214 msec , p=0.005 ) significantly decreased . 
 there were no significant changes in the pr and rr intervals , as well as the qrs duration . 
 our findings suggest that the thyroid hormone affects ventricular inhomogenicity , and that l - thyroxine replacement therapy may reduce malignant ventricular arrhythmia and sudden cardiac death in primary hypothyroidism . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
Clinical characteristics
Baseline ECG
Electrocardiographic parameter
DISCUSSION