For girls, I find the most extreme behavior occurs around age 11-12 and then eases up a bit. This tends to be delayed in boys by a year or two. I dealt with my now ex's daughter (undiagnosed) during these years and it was quite frankly a nightmare. My cousin had boys with ADHD and I would describe the age 12-13 behavior of one of them as nothing short of abusesive towards the mom. So yes, there is a hormonal or life stage aspect to this. These are cases of very extreme behavior. Although the behavior tends to become milder in later years, you then have all the teenage issues to deal with.
Whatever parent was the child's favorite will receive the greatest level of abuse and will be the one the child will least listen to. Parents are now the enemy and the favorite one is at the top of the list. Often children will behave fairly well outside the home, so you can expect the outside world to have no idea of what you are dealing with and will direct the blame on the parents.
In the case of the 11 year old I dealt with, her biological father took her right up to the doors of an all girls religious school to register her. She backed down a lot and it solved some of the school problems. I don't believe in threats, but it did work to some degree. If you can stick to it, you may consider cutting her off everything she likes for long enough to get through to her. This however can be a 24/7 effort on your part for a good month before it begins to work. It also needs everyone involved to cooperate on this plan (rarely does this happen). Sometimes another adult less familiar to the child can get through to them.
You may find extreme changes in talent during this stage. Hard to say which way and in what subjects you will see the changes.
It would not hurt to look into diet and sleep as others have mentioned, further psychological evaluation and medical testing. Don't get your hopes up on finding a solution through further evaluation any time soon. I am no expert, just a person with some experience.
