Shamrogue,
It is a law of physics, that air gets less dense when hot and therefore as temperatures rise aircraft get less lift. Any aircraft will have performance charts for a particualr runway and as the temeperatures rise the aircraft will be able to lift less of the maximum take off weight from that runway.
Then depending on the lenght of the sector to be flown, the crew may have a choice to make to either reduce the payload or ulpift less fuel and tech-stop en-route.