MWM,
Dylan's thyroid issues started from the Lithium. His TSH was an incredible 107, very, very high.
Our psychiatrist did a microbody check with the rest of the bloodwork when he first checked his thyroid. From what I understood (in non-doctor terms) the microbody check would tell us if Dylan would have eventually had a thyroid issue later in life, with or without the Lithium. The microbody check came back positive. So, basically, the Lithium sparked it, but it would have been a problem at some point anyway.
Dylan did not seem overly hungry when the thyroid issue sparked, however, he was very lazy. I couldn't get him off the sofa, couldn't get him out to play, ride his bike, none of that. Once we got his TSH level down to normal (treating him with Synthroid), his energy levels went back up. Keep in mind, Dylan's no athlete, but at least he has a desire to go outside and do things now, whereas before he didn't.
My normal 15 year old is an eater. I can tell when he's going to sprout again, because as soon as dinner is over he's looking in the cupboards. He's a huge carb eater too. And he doesn't have a thyroid issue.
You could just ask the family doctor to do a simple blood draw for the thyroid. Think you need to fast before (or maybe that was for the Lithium, I dunno). Anyway ~ couldn't hurt to check.