In formal writing, thru is not an acceptable substitute for through.  Despite the widespread acceptance of thruway as a variant of throughway, thru is unacceptable and suggests that the writer is too lazy to type or write the three extra letters in the more conventional form of the word.

The development of phonic shorthand in Internet chatrooms and instant messaging has caused some people to adopt thru (and other phonic spellings such as tho and altho) in more traditional forms of writing, but that does not make these spellings acceptable in formal and semiformal writing.  Chat and instant messaging require speed; therefore, some shortcuts may be pardoned there.  However, the recipient of a letter or report, or the reader of an essay, has every right to expect that the author took the time to develop a carefully considered message.  Abbreviated forms and phonic spelling send a signal that the message was written hastily and is not carefully thought-out.