The following are extracts from the Scientific American website in an "In Focus" article, entitled "Paying Attention". Please visit their website for the full article.
"......Alan Zametkin of the National Institute of Mental Health (NIMH) has found, that a small subset of ADHD people have a different receptor for thyroid hormone and that 70 to 80 percent of all people with this very rare difference in their thyroid receptor have ADHD.
Other studies have found an association between ADHD and three genes encoding receptors for the neurotransmitter dopamine......
Neurochemistry is not the whole story. Scientists have also discovered structural abnormalities. F. Xavier Castellanos of the NIMH used magnetic resonance imaging to measure the total brain volume and several different brain regions in 57 ADHD boys and 55 healthy control subjects. His team found that the anterior frontal part of the brain was on average more than 5 percent smaller on the right side in ADHD boys. The right caudate and the globus pallidus, too, were smaller. These structures form the main neural circuit by which the cortex inhibits behavior, and so damage there might well manifest itself as a lack of impulse control. Castellanos warns that this result offers but part of the puzzle: "It's only slightly better than phrenology. Now we're just measuring the bumps on the inside of the brain."
Another facet of ADHD malfunctioning comes from positron emission tomography (PET) studies. Julie B. Schweitzer of Emory University monitored brain activity in ADHD and unaffected men while they completed a task. Participants heard a series of numbers, one every 2.4 seconds, and were asked to add the last two digits they heard. Looking at the PET scans, Schweitzer saw two major differences between the groups. First, the ADHD individuals maintained high levels of blood fiow, whereas the controls displayed deactivation in the temporal gyrus region-indicating some kind of learning."