I had a lad in my class, at around your age, who never said a word to anyone. He wouldn't even answer his name when I called the register. I was amazed to find out that he chatted freely to his parents and neighbours, and that he'd conversed normally at school earlier in his life.
I suggest writing to your doctor about the problem (since talking to him/her, at the surgery, is bound to be a problem for you). A decent doctor should be prepared to visit you at home (or to get a specialist to do so). If you don't get any help though, don't just give up. You might find literally shouting out difficult but that shouldn't prevent you from metaphorically doing so (via letter and emails). Keep pushing for help until you get it!
Remember as well though, that hormonal changes mean that many teenagers find odd things going on in their lives. It might be 'just a phase' which you'll possibly 'grow out of' anyway.
Chris