You should definately see an ENT. I lost my voice for 6 weeks after surgery, even though my vocal cords were not damaged during surgery. It sounds like it needs to be done. My tumor was also large 4.8cm.
I had my thyroid removed a week ago today, and I have had zero problems with my voice. I had a bad sore throat for a bit from the breathing tube during surgery, but that was really it. I do local community theatre and was worried about my voice, as well, and the doctor (general surgeon for me) he couldn't guarantee anything, of course. I'm not sure how rare of an effect it is, though. It really can be the luck of the draw, I think.
It is a terrible situation to be in, and I wish you the best of luck with it!
If you have concerns ask around and find the very best surgeon in your area. When I had my TT, I had a great surgeon who specialzed in chest and throat surgery. He used some kind of a nerve stemulator during surgery to help guide around the major nerves in that area. The hospital required a signed consent for its use. That is what brought it to my attention. I'm about 4 mos. out from my surgery and my advise is do nothing to strain your voice for a period of time. Mine was fine afer surgery,maybe a little week but I got a bad respatory infection with a lot of coughing and I am a Portrait Photographer. About that same time I was trying to get the attention of a screamming 2 year old. It did not take long for me to see the foolishness of that move. At this point my voice tires easily and is maybe a little lower than it was and no I can's sing, but than again as my husband tells me I could not sing before either. So remember your health is the most important consideration, and we are all here for you.