For support Thresh it is important to make sure your final items have cool down reductions. You are not the carry for the team nor the person who does the damage. Use your chain to lock the enemy in and let you partner make the killing blow. Items may vary depending on foes.

I took alot of info on the items from Gems are Truly Outragous

Just convert the info to Thresh. Hes like Blitzcrank on buddy crack.