Nevertheless, a proportion of adults who meet the criteria for ADHD in adulthood would not have been diagnosed with ADHD as children. Most cases of late-onset ADHD develop the disorder between the ages of 12-16 and may therefore be considered early adult or adolescent-onset ADHD. Differential diagnosis
The DSM provides potential differential diagnoses - potential alternate explanations for specific symptoms. Assessment and investigation of clinical history determines which is the most appropriate diagnosis. The DSM-5 suggests ODD, intermittent explosive disorder, and other neurodevelopmental disorders (such as stereotypic movement disorder and Tourettes disorder), in addition to specific learning disorder, intellectual developmental disorder, ASD, reactive attachment disorder, anxiety disorders, depressive disorders, bipolar disorder, disruptive mood dysregulation disorder, substance use disorder, personality disorders, psychotic disorders, medication-induced symptoms, and neurocognitive disorders. Many but not all of these are also common comorbidities of ADHD. The DSM-5-TR also suggests post-traumatic stress disorder.Symptoms of ADHD, such as low mood and poor self-image, mood swings, and irritability, can be confused with dysthymia, cyclothymia or bipolar disorder as well as with borderline personality disorder. : 10  Some symptoms that are due to anxiety disorders, personality disorder, developmental disabilities or intellectual disability or the effects of substance abuse such as intoxication and withdrawal can overlap with ADHD. These disorders can also sometimes occur along with ADHD. Medical conditions which can cause ADHD-type symptoms include: hyperthyroidism, seizure disorder, lead toxicity, hearing deficits, hepatic disease, sleep apnea, drug interactions, untreated celiac disease, and head injury.Primary sleep disorders may affect attention and behavior and the symptoms of ADHD may affect sleep.
Oppositional defiant disorder (ODD) occurs in about 25% of children with an inattentive presentation and 50% of those with a combined presentation. It is characterised by angry or irritable mood, argumentative or defiant behavior and vindictiveness which are age-inappropriate. Conduct disorder (CD) occurs in about 25% of adolescents with ADHD. It is characterised by aggression, destruction of property, deceitfulness, theft and violations of rules. Adolescents with ADHD who also have CD are more likely to develop antisocial personality disorder in adulthood. Brain imaging supports that CD and ADHD are separate conditions, wherein conduct disorder was shown to reduce the size of ones temporal lobe and limbic system, and increase the size of ones orbitofrontal cortex, whereas ADHD was shown to reduce connections in the cerebellum and prefrontal cortex more broadly. Conduct disorder involves more impairment in motivation control than ADHD. Intermittent explosive disorder is characterised by sudden and disproportionate outbursts of anger and co-occurs in individuals with ADHD more frequently than in the general population.Anxiety and mood disorders are frequent comorbidities. Anxiety disorders have been found to occur more commonly in the ADHD population, as have mood disorders (especially bipolar disorder and major depressive disorder). Boys diagnosed with the combined ADHD subtype are more likely to have a mood disorder. Adults and children with ADHD sometimes also have bipolar disorder, which requires careful assessment to accurately diagnose and treat both conditions.Sleep disorders and ADHD commonly co-exist. They can also occur as a side effect of medications used to treat ADHD.