There could be a number of things in play here. People do, in some cases, "outgrow" ADHD or otherwise see a reduction in their symptoms; so one drug regimen could have nothing to do with the other. I have heard of cases where the psychological symptoms of GID mimic ADHD, once HRT is started the symptoms resolve themselves.
Talk to both doctors about your concerns, I don't think its a drug interaction but a lessening of symptoms.