the laws that protect you from job termination differ from country to country-- and we dont know where you live so i cant begin to advise you.
i live in the usa where we have a law called the americans with disabilities act that was signed in 1992 by george h w bush. i dont think they can legally fire you for seeking treatment for a mental health problem. if you are really worried then you should consider lying about the reasons for your hospitalization.
good luck
pi