The interjection is "Thank God!". However if you are speaking to (or praying to) God, you would say Thanks God, in this case you are expressing your gratitude

The phrase “Thank God” is very common in the US. It can be used in so many ways when talking. For example, “Thank god, the weather is getting hotter!” It is usually said when there’s some kind of excitement.

This is a common mistake I have seen. In U.S. English, "thank God" is correct. "Thank God you're here; I wouldn't know what to do without you."

Thanks to everyone!