I work with a lady who had undiagnosed low thyroid for YEARS. She's one of those border collie types that just has too much pent-up energy to sit down and focus on anything unless she runs 6+ miles in the morning. So that's what she does, every day. Of course she was slim and trim with no signs of thyroid issues. They only caught it last year because she started having other issues and turns out it was cancer. Now she has no thyroid and MINIMAL meds because, you guessed it, she runs the equivalent of 2 marathons every week.