Children with ADHD are far more likely than average to express gender variance.
A study on children  found that, as compared to non-referred comparisons, participants with ADHD were 6.64 times more likely to express gender variance than participants without ADHD. The study also found that, in cases of ADHD, this gender variance was related to elevated emotional symptoms.
 Strang, J.F., Kenworthy, L., Dominska, A., Sokoloff, J., Kenealy, L.E., Berl, M., Walsh, K., Menvielle, E., Slesaransky-Poe, G., Kim, K.E., Luong-Tran, C., Meagher, H. & Wallace, G.L. (2014) Increased gender variance in autism spectrum disorders and attention deficit hyperactivity disorder. Arch Sex Behav 43 (8): 1525-33. [Link]