The common abbreviation in texting, for example, is THX. You can also write TY for "thank you." Language evolves. Although these uses are not acceptable in formal or business communications, they are widely used in electronic and casual communications today.

I agree with Joseph on this. Unless you are sending a text message, or writing an informal note to a friend, you should stick with "Thanks." But in situations when you really want to be casual, "Thx" is okay.

since these are both non standard abbreviations, both would be acceptable, but you cannnot use them in any formal situations.