Slowed Down

I started serious running when I was 9 and achieved alot. I ran at nationals 3km cross country and placed tenth when I was 11 years old. In high school I came 1st in state 800m finals. I also came 7th in national 800m that same year. I was diagnosed with hypothyroidism when I was twelve years old (which was unusual for such a young person) and a dramatic decline in my running soon followed. I am now 14 years old and can barely place at regionals let alone make it to state. My mum thinks it might be from this disease but doesn't want me to think in that mindset. My endocrinologist is really great but he's not sure. I just want to know if it can affect that sort of thing. has anyone else experienced a similar problem?