User blog comment:TheTobarMethod/AD Thresh/@comment-207.237.44.252-20130125010642

Everything else in his kit revolves around AP, even the active part of his Q. His Q passive also requires a wind up or wait between attacks, so while on top, poking will not be as effective as you want it to be early game. In order for you to do well with AD build late game, AS and lifesteal will also be necessary, and then suddenly you are straying away from the point of his Q. Any AS whatsoever is not necessary (though Nashor's tooth and Icy Mallet would be good items, though you do not mention them)

No hate, more like AD is not a good option on him. If he is lane sharing support on bot, he should be paired with somebody with high poke, or low ranged burst (fiora, xin, etc.) They need his Dark Passage more than a slightly more damaging poke early game. By the time mid game comes, poking isn't needed anymore, so you'd have a bunch of wasted AD items, and you would be less of a threat when you utilize his W Q R E combo due to less shields and less magic damage. If you play Thresh, go for a nearly pure AP, Armor/MR, Health build, because you want attention in teamfights.

Thresh isn't a good carry.