© TÜBİTAK.Background/aim: Primary hyperparathyroidism (PHPT) is an endocrine disorder characterized by hypercalcemia caused by excessive parathyroid hormone (PTH) secretion from the parathyroid gland. PHPT was previously shown to increase cardiac arrhythmias. Besides, new indices, such as the Tpeak-Tend (Tp-e) interval, Tp-e interval/QT interval (Tp-e/QT) ratio, and Tp-e interval/corrected QT interval (Tp-e/QTc) ratio may be associated with ventricular arrhythmias and sudden cardiac death. Therefore, we aimed to investigate the relationship between PHPT and the changes to Tp-e interval, Tp-e/QT ratio, and Tp-e/QTc ratio. Materials and methods: We carried out the study with 41 patients with PHPT and 40 control subjects. We calculated the Tp-e interval, Tp-e/QT ratio, and Tp-e/QTc ratio of the participants from the V5 derivations on their ECG papers. While we defined Tp-e interval as the distance between the peak and the end of the T wave, Tp-e/QT and Tp-e/QTc ratios were calculated by dividing Tp-e by QT and Tp-e by QTc, respectively. Results: Total calcium, albumin-corrected calcium, phosphorus, and PTH levels were significantly higher in patients with PHPT. We also found positive correlations between albumin-corrected calcium and PTH levels and Tp-e interval, Tp-e/QT ratio, and Tp-e/QTc ratio (p < 0.001). Conclusion: Our results suggest that Tp-e may enhance the current knowledge on arrhythmic risk in PHPT patients better than basal ECG. In addition, both high PTH and high calcium levels appear to have the potential to cause arrhythmogenic effects.