Mine was noticed because I have a multinodular goider. I asked about potential issues with it a couple times over the years and was always told I just needed to lose weight till the doctor finally noticed it in my early 20s. Who knew it was a lot easier to be active and control your appetite when your metabolism is actually working and you don't feel like you always wanna crawl into bed?
You normally will get it after puberty, pregnancy, or menopause(at least with what I have, which is hashimotos thyroiditis, an autoimmune disease. Not sure about others) so, if it's similar you likely developed it during puberty like myself and didn't realize it or notice the changes because you assumed it was normal.
I worked for a brewery that brought in all their water from the same spring or something. Even though it was a chain with multiple us locations.