Why could my thyroid be enlarged?

I was visting my endocrinologist today for an unrelated issue when he noticed my thyroid was enlarged. He looked at it for a while and then asked a doctor for a second opinion. She also felt it was definitely enlarged. They also mentioned my neck felt full. When I had a full battery of tests two years ago with another endo, my thyroid results were fine. I am 18 year old male btw. Any ideas for the cause? I am having a bunch of tests done tomorrow including a full thyroid work up.