Though is used as an adverb as the less formal equivalent of however. We use though and however when we want to add a comment that seems to contradict or contrasts with what has already been said. Though often indicates an afterthought.
"Tho" is not textspeak. It has existed for hundreds of years and saw a spike in usage in the 19th century.
See here: https://www.merriam-webster.com/dictionary/tho
And here: https://bit.ly/2K3hx6H
"Wen u gettin home tho?"
"When are you getting home though?"
Avoid using it.