E.T.
I don't know about a special needs child or the hormones, but 9.5 isn't that early to be showing signs of puberty.
I had breasts in 4th grade (9 years old) and got my cycle at 10.
I would think the synthroid is just letting her develop as she is supposed to.
My bet is the defiance is her asserting her independence which is a normal part of puberty. She sounds normal.