As a closeted trans (FTM) with crazy transphobic christian parents myself, I would tell them that you have anxiety that makes you sick and that you need help. If they ask you why, do not, and I repeat, DO NOT TELL THEM THE TRUTH. They will freak out and not take you seriously (or worse, try to "fix" you) if they are anything like my parents. Make up something if you have to (school, work, friends, etc). Hope this helps friend! I'll try to help out in any other way I can. Us guys gotta stick together!