There's a lot of comorbidity between ADHD and stuttering, so it's fairly common. They're both related to dopaminergic systems in the prefrontal cortex but my understanding is they are different pathways in the cortex, but it's been a few years since I was heavy into neuro research so I might be off a bit. I got diagnosed with ADHD at 21 (I'm now 30), and after getting on the right meds (concerta for me) my grades in university went from 70% average to 93% average. It also significant improved my speech fluency, which I'm still not sure how that all works. My hypothesis is that because Im able to focus so much better with medication, I'm able to split my attention between all the speech fluency tricks I know and then keep the conversation going at the same time. Where previously my brain would be going to 50 different locations at once and none of them would be relevant to the topic at hand, but then I'd hit a hard block and all my attention would be yanked back to said block. All this to say that the diagnosis and resultant medication was genuinely the best thing that ever happened to me as it made every aspect of my life easier. I became so much more outgoing because my speech was better but also because I felt a lot better about myself because I wasnt constantly procrastinating due to ADHD