Application of nanofluids as a working fluid can increase the thermal efficiency of heat pipes. The objective of this comprehensive review is to study the previous works to understand the dominant mechanisms of heat transfer when nanofluids are used in thermosyphon heat pipe (THP). The influence of heat input, type and concentration of nanoparticles on the thermal behavior of THPs are investigated. In general, nanofluids significantly improve the thermal characteristics of THP by decreasing the thermal resistance and increase the thermal efficiency of THP. The thermal performance of THP strongly depends on operating variables, especially the type of nanoparticles, their concentration and addition of surfactant and heat inputs. Further, the thermal performance of THP at various heat input is remarkably related to the type of nanofluid. This study also presents a review of the applications of the nanofluid-based heat pipes in energy systems. Finally, some challenges are identified and suggestions are made to fertilize the future investigations.