In the first fight against the undead on Sunday's episode of Game of Thrones, Jon Snow and the others were attacked by a wight polar bear of all things. And after one of the fighters in their group died from the attack, Thoros was injured too. But is Thoros a wight now on Game of Thrones? He survived the wight polar bear attack initially, but dying north of The Wall can often result in the dead turning into wights.
Either the Night King or his White Walker companions can turn any dead human or animal into a wight to use as a foot soldier, so to speak, and if Thoros' body wasn't burned, he could have been turned as well. Luckily, there were enough northern men in Jon's group to know that you always burn a body north of The Wall or you risk the chance of the Night King or a White Walker turning them into a wight for their growing army.
So since Thoros is dead on Game of Thrones and isn't going to be taken back to Westeros south of The Wall, he won't be turning into a wight anytime soon. Unfortunately, this means that a solid member of the Brotherhood without Banners is now dead and that Beric Dondarrion doesn't have the companion who has brought him back from the dead numerous times.
Just like the Lord of Light supposedly told Melisandre to bring Jon back from the dead, Thoros, a red priest, has brought Beric back from the dead more than once to serve the Lord of Light. Thoros has also fought with Beric for years and has acted as his personal red priest, in a way. Without Thoros here to bring Beric back from the dead in the future, it looks like the experienced fighter will have to live his best life since this is now the last one he has.
For Season 7 and really the series as a whole, Thoros served his purpose. He's not only been a big part of the Brotherhood without Banners, but he also helped the Hound see his vision in the flames. The vision of the mountain shaped like an arrowhead helped lead them to the army of the dead, but looking into the fire at all was a pretty big feat for the Hound.
Thoros was a huge part of that, and since we obviously couldn't lose anyone else just yet, he was unfortunately the most expendable character at the moment. Luckily though, since Jon and company were sure to burn his body after his death, they won't be forced to fight a reanimated Thoros in the war to come.