Core symptoms of ADHD are inattention (J's target - or one of the many - in the first year of school was to be able to concentrate on one thing for 2 minutes, and he never managed it; he's also completely oblivious to instructions in school because he's so distracted by everything around him) impulsivity (doing things without thinking first - he still shouts out all the time at school, age 7, but then so the the 15 year old kids who I teach who have ADHD!) and hyperactivity (J literally never stopped moving from the moment he could crawl unless he was strapped into a car or aeroplane or pushchair).
J's massive hyperactivity masked the autism for ages because he never stopped long enough for anyone to notice the social deficits.
J was dx ADHD at 4 officially but the paed we saw when J was 3 said he was sure it was ADHD plus something else. That paed couldn't dx as only CAMHS can here and CAMHS only take 5 year olds - we got seen early but still waited for an official dx for over a year. Didn't make any difference though tbh, as I knew it was ADHD, didn't want medication for him and he was getting a Statement anyway without a dx.
But then the hyperactivity, which had always been accompanied by very violent and dangerous behaviour, got out of control at school in his 3rd year there, so we went back to CAMHS to ask for medication. The new psychiatrist spotted the autism and said that there's a real debate about whether ADHD and autism are separate conditions, as children with autism will often have similar symptoms as part of their autism e.g. impulsive and hyperactive because they don't 'get'/ care about social norms or others' expectations of them and inattentive because of sensory issues associated with autism.
So J's dx of ADHD remains because it was there first, but the autism is the primary dx and 'trumps' the ADHD in diagnostic terms i.e. if he'd been dx'd autistic first, there would have been no additional ADHD dx because autism accounts for those symptoms too. Hope that makes sense!
Have to say, the medication trial has helped to calm him and improve his concentration and reduce his hyperactivity. Now the autism is much more obvious because he stays in one place long enough to notice! He's on Straterra which is an ADHD med which isn't a stimulant like Ritalin (which stimulates dopamine, I think), but encourages noradrenaline (I think - could be the other way round, but I know it stimulates something else and works in a different way!)
Apparently this one is very good for those with ADHD (or symptoms of it as part of autism) and the psych says her autistic patients do well on it. Helped his sleep and general calmness and I wish it wasn't necessary but it was!
Probably a longer answer than needed but HTH!