“Thank God” is actually a shortened version of “Thanks be to God” which means that you are telling the others (your friends or whoever) that you are thankful to God.
“Thank God” is in the same way as “Praise God”.
“Thank God” is also correct because you are using “Thank” as a verb I (we) thank God.
You are praying or directly conversing with God, then you would say “Thanks, God”. “Thanks, god” should actually mean “Thank You God” if he was in front of you.
So, the correct expression is: Thank God It’s Friday or, commonly known as TGIF. Its origin dates to the 1960s. It was first popularized by the restaurant T.G.I. Friday’s which was founded in 1965. However, it became mainstream by the late 1970s mainly due to the 1978 movie of the same name. It is also an acronym for Toes Go In First, which is the punch-line of various blonde jokes.