Can you prove that? Because my understanding is that ADHD isn't diagnosable by imaging the brain. If there was a structural difference, presumably it would show up on scans? Or at least post-mortems?
AFAIK it is visible on fMRI, but doing so is incredibly expensive and taxing on the person - you'd need to keep them in the fMRI for very long time, repeatedly, and watch them doing different tasks.
But I've seen papers about this where they did that with some people. The brain simply doesn't fire up like a normal brain does. The structural difference itself might be too small (on cellular or even molecular level) to see with current resolution, but the effect is clearly visible - as well as that medication makes the brain behave more like normal brains.
Quick search:
- https://www.medicalnewstoday.com/articles/315884
- https://bmcmedimaging.biomedcentral.com/articles/10.1186/s12...
The ability to recognise when someone might be thirsty, doesn't mean dehydration no longer exists. They go together.