hello and welcome to my guide on building thresh as an ad carry, yes you heard right.
in this guide i will be covering the ideas behind this build, why it works.

the runes are pretty standard, armor pen reds and flat ad quints for extra early game damage, flat armour yellows for some early game sustain and scaling magic resist to help out later in the game.

for masteries ive gone with a pretty standard 21/0/9 set but with one of the armor pen masteries dropped to give him spellblade, this will give him a bit of extra punch late game. i also made sure to pick up the total biscuit mastery to give him sustain in lane while still going dorans blade first.

as adc thresh you are focused on farming early game just like any other ad carry but dont neglect souls as they are a vital part of scaling into late game.
the thing that makes this style of play work is the passive effect on his q, which adds the number of souls to his attacks as well as a portion of his attack damage based on how long since he last attacked. this means he can store up 1 huge hit that will knock most of the health off your opponent quickly