My younger brother is three, and what I have read online seems to fit the profile. He's my half brother, and I havent mentioned anything to my father or his wife, as I do not want to upset them. he has speech delay, and seems to me like he has some delayed cognitive development also (as far as understanding reversibility) and has SEVERE seperation anxiety.

What ADHD symptoms did your preeschooler exhibit, (assuming they were diagnosed formally)?

Some things might be:
Overly active, compared to other children his age. sometimes, more irritable than would normally be expected. Over-reacts to things, at times. Runs around, not listening to what they are being told, and more defiant than might be expected if the parents are doing what they should be doing. sometimes they get more infections than normal (according to specific authorities, but no all agree with this). Poor sleep patterns, sometimes not seeming to be able to sleep very well. these are some of the things my son experienced as a toddler. he would think it was funny to knock the blocks down that I had just tried to help him stack. The knocking down was more fun than the experience of stacking them. this might just be normal toddler-hood, though. One thing you might have them look into is the effects of mercury on behavior and development (from vaccinations), which is easy to look up online, and many books are available. My son did have a problem with separation anxiety, also, and I believe that was due to the sensitivty to many things that is often seen with ADHD. he would even complain about how his socks felt, inside his shoes. If there were any wrinkles, this was a major problem, as they seem to see, hear, sense and feel things more sensitively than others might normally experience. Their senses are "more alert". One book that explains this is, "Allergies and the Hyperactive Child", by Dr. Doris Rapp. I think it might also touch on the mercury issue, but I am not sure. (Used books, online). I, personally, disagree that ADHD does not show some sypmpotms, sometimes before school age. it is just not always noticed. That does not mean that every toddler that is running around has sypmtoms of ADHD, it just means that there are some very specific things that can, sometimes, be seen.