I think I may have thyroid disease, I'm a 15 year old male, 6'1 243 lbs. I eat less than most of my friends yet they are all fit

I hit the gym 3 times a week and I go for a 6-8 mile bike ride 3 times a week also.

what kind of a doctor would I go to to check this? and, supposing thyroid was my problem what would the treatment be? how well would it work?