Through and through is an adverb and means to do something completely, in any manner possible. A good synonym is thoroughly. Sometimes this phrase is used as an adjective with regard to bullet wounds. A through and through is something that goes in one side of something or someone and comes out the other side, as opposed to staying inside the body or object.
Thru is an alternative spelling to through and has its own listing in the dictionary. Though some European dictionaries list it as mainly being used in North America. This alternate spelling is used somewhat in the form thru and thru, but it is vastly outnumbered in usage with through and through. Most of the instances of it we found were in lesser quality publications or extremely informal settings, like the comment section of articles.
One exception seems to be in headlines. We saw one or two instances when the shorter spelling was preferred in a headline to keep the word count down. The downside to this strategy is that the article automatically seems less formal and more commonplace.
The discussion of the differences between through and thru are discussed more in depth in another article.
At first, Stas considered making a smartwatch through and through — LCD screen, apps, Google’s free operating system for wearable devices. [The New York Times Magazine]
Xavi is Barcelona through and through, joining the Catalan club’s prestigious La Masia youth set-up at 11 and going on to win eight La Liga titles, three Champions League trophies and becoming an integral part of the Spain team that won the 2010 World Cup and the past two European Championships. [The National]
The bullet that hit her was a through and through, so surgery was not necessary. [WTVM]