Updated for patch 4.3.
There are a few types of Thorium Veins you will encounter on these routes, small and rich. You must be mining skill 230 to mine the Small Thorium veins and mining skill 255 to mine the Rich Thorium Veins. If the reason you are mining is for gold then please take a look at this Gold Guide which teaches you how to reach the gold cap.
The two zones I have chosen are Winterspring and Silithus which both have a mix of each of the Thorium veins.
Thorium Ore in Wintersrping
This is what I consider to be the best zone for farming Thorium in. Go into the caves for more farming spots along the route. You shouldn’t have a problem collecting the ore you need in this zone.
Thorium Ore in Silithus
This is a really simple route to run, just go in a big circle or square around the entire zone. You can go into the tunnels for even more Thorium Ore farming opportunities.
If you’re looking for information on Leveling your Mining see my WoW Mining Guide.