Background: Several studies in adults, showed increased dispersion of QT and corrected QT (QTc), peak-to-end interval of the T wave (Tp-e), Tp-e/QT ratio and Tp-e/QTc ratio in hypothyroidism. However, there were few studies in children with hypothyroidism. Objective: This study aimed to evaluate the relationship between hypothyroidism in children and increased QTd, QTc, Tp-e, Tp-e/QT ratio and Tp-e/QTc ratio. zPatients and methods: In this study we compared 50 hypothyroid children with 30 healthy children in respect of serum thyroid stimulating hormone (TSH), serum level of free triiodothyronine (fT3) and level of free thyroxine (fT4). Hypothyroidism diagnosed with TSH above the laboratory accepted upper limit and normal or decreased fT4 values. The patient and control group data were compared by calculating the QT interval, corrected QT(QTc), QT dispersion (QTd), QTc dispersion (QTcd), Tp-e, Tp-e/QT ratio and Tp-e/QTc ratio on 12-lead surface electrocardiogram (ECG). Results: In the hypothyroid group, the mean age was 10.36 ± 4.18 years old (ranged from 1-17 years). Regarding the control group, the mean age was 9.09 ± 4.58 years old (ranged from 1-17 years). The QT max, QTc max, QTd, QTcd, Tp-e, Tp-e/QT and Tp-e/QTc values were determined to be significantly higher in hypothyroid children than the control children (p=0.03, p=0.026, p< 0.001, p=0.004, p=0.003, p=0.034 and p=0.027 respectively). QT max, QTc max, QTd and QTcd in the hypothyroid group were 330 ± 38 ms, 406 ± 26 ms, 37 ± 17 ms and 39 ± 17 ms respectively. While in the control group, they were 315 ± 23 ms, 393 ± 26 ms, 18± 13 and 27 ± 16 respectively. Conclusion: It could be concluded that there was a significant increase in QTd, QTcd, Tp-e, Tp-e/QT ratio and Tp-e/QTc ratio values in children with hypothyroidism similar to that seen in adults. The use of 12- lead ECG for the measurements of QTd, QTcd and Tp-e can be considered necessary for the monitoring of children with hypothyroidism and in the evaluation of the risk of cardiovascular disease.