ADHD symptoms are far more noticeable (despite being missed as often as they are). *Subclinical* thyroid hormone fluctuations in childhood would lead to symptoms as subtle as crowded teeth, repeated cavities despite good oral care, or a shorter than expected height. Nobody sees these things and thinks "thyroid imbalance!" because it doesn't have an immediate impact on quality of life and so doctors aren't on the watch for it. But since it's a progressive disease, eventually the more well-known symptoms catch up with you as well.

Anyway as the theory goes, it's a person's mother having untreated thyroid disease during pregnancy that would lead to ADHD in those with the genetic predisposition for it. You still have to have the genes for ADHD in the first place, in order for them to be expressed. (Incidentally thyroid hormone is *also* a regulator of gene expression.)

So I'm not trying to say all cases of ADHD are caused by thyroid hormone imbalance or that mothers with untreated thyroid imbalance will always have ADHD kids. Just saying there appears to be quite a case brewing for it to be a significant contributor.