An overactive thyroid is not the likely cause of your pain. Yes, you can experience aches and pains from thyroid issues, but not typically what you're describing. I suggest you visit a local chiropractor to have the neck fully examined to find out the actual cause.
Beat of luck,
My suggestion would be to see a local chiropractor to get your spine checked. Even though it was a low speed impact, there is a lot of trauma that happens to the body. Many times, symptoms don't show up for days or weeks later. Getting your spine back in proper alignment along with some muscle work will go a long way in helping you feel better.