I mean abnormal brain in the most obvious way possible. A brain that, by way of genetic variation or injury, deviates from what is average.
A person with hypothryoidism has a statistically uncommon brain, one that produces a statistically uncommon amount of thyroid hormone which can result in statistically uncommon emotional responses to stimuli, which usually results in statistically uncommon behavior in a fixed environment. Statistically.
If you need further clarification, let me know.