The frequency and intensity of extreme high temperature(EHT) in the northern hemisphere exhibit remarkable low-frequency(LF) variations in summer over the time period 1951-2017. Seven hotspots featuring both a high occurrence of EHT and large LF variations in EHT were identified in the northern hemisphere. The probability density functions show that the higher probabilities of the occurrence of EHT over these hotspots in recent decades are consistent with the shift in the mean temperature and an increased variance in the daily mean temperature. The common features of the LF variation in EHT frequency over all domains are a remarkable increasing trend and an evident decadal variations(DV). The DV component makes the main contribution to the large LF variations. Further analysis shows that the DV over the six hotspots are the footprints of two dominant natural internal signals: the Atlantic Multi-decadal Oscillation(AMO) and the Inter-decadal Pacific Oscillation(IPO). The coherent variation in temperature over western North America-Mexico, eastern Siberia, Europe, the Mongolian Plateau and southeastern China is significantly influenced by the AMO. The AMO contributes to the variations in temperature over the first four hotspots via its associated barotropic circum-global teleconnection, which imposes a striking anomalous pressure over these regions. The IPO governs the DV in temperature over the Indo-China Peninsula through an anomalous Walker circulation driven by the IPO-associated sea surface temperatures in the tropical Indian Ocean. The inconsistency between the in situ observations and the reanalysis datasets causes the uncertainty in the physical linkage between the natural internal variability and variations in EHT over southeastern China and the Indo-China Peninsula. This implies that natural internal forcing plays an important role in making hotspots more vulnerable to EHT.