In addition, "go to the hospital" is AE for "go to hospital" in the sense that cropje jnr describes. Americans go to the hospital, even when no particular hospital is meant; "go to hospital" (no "the") is not used in AE.
This has been discussed many times before. If you put "preposition hospital" in the dictionary search box above, you will get this list of threads. They are very interesting.
Generally, AE speakers always include the prepositions ("a" or "the"), while other English speakers including BE do not unless they are speaking about a specific hospital (as Cropje explained). I didn't know about that difference before I happened upon this forum.
As far as I know, if you go to or are in the hospital, that means you have to receive some medical treatment there. However, when you go to (a) hospital, it means that you either work in a hospital or pay a visit there.