Many psychiatric conditions — including anxiety, depression, substance abuse, and personality disorders — mimic or mask the symptoms of adult ADHD. Sometimes it's hard for a medical expert to know which disease or condition is causing which symptoms and how to best treat them.

A high percentage of adults with ADHD also suffer from comorbid disorders that commonly occur alongside adult ADHD, and which may exacerbate a physician's attempt to isolate and treat symptoms. The most common overlapping diseases and conditions include clinical depression and anxiety, bipolar disorder, substance abuse, alcoholism, learning disorders, dyslexia, fibromyalgia, brain injuries, dementia, psychosis, hypothyroidism, hyperthyroidism, conduct disorder, speech and communication problems, sensory integration disorders, oppositional defiance disorder, and sleep disorders.

People with untreated thyroid conditions, including hypothyroidism and hyperthyroidism, often display symptoms presented in adult ADHD. Individuals who are hypothyroid (the thyroid is too slow) are often depressed, lethargic, and disinterested, while those who are hyperthyroid (the thyroid is too fast) tend to be irritable, nervous, anxious, agitated, and excitable.

Research also shows that adults with ADHD are affected by many other problems, including erratic moods, rollercoaster emotions, a short temper, chronic pessimism, a constant craving for stimulation, a desire not to be touched, highly fluctuating energy levels, clumsiness, and problems with hand-eye coordination. Psychotics are sometimes able to convince friends, family, and even their medical doctors that their real problem is undiagnosed and untreated adult ADHD.