The cause of ADHD is unknown. It is unlikely that a single etiologic
factor leads to all cases of the clinical syndrome of ADHD. Most likely,
an interplay of both psychosocial and biological factors exists that can
lead to the syndrome of ADHD. Some conditions such as fetal alcohol
syndrome, fragile X syndrome, low birth weight, and very rare,
genetically transmitted thyroid disorders can present behaviorally with
symptoms of ADHD. However, these cases make up only a small portion of
the total population of children with the disorder.