I understand ADHD is a standard term, but I'm still a bit suspicious that it's not a useful one; Psychiatry doesn't seem like the most reliable field
Okay, I know this is going to sound weird, but I think ADHD is another name for hypofrontality-mediated unfulfilled potential
ADHD as "unfulfilled potential"
Although it is characterised by a dysregulation of executive function, if you look at the diagnostic criteria for ADHD, it's all about a functional impairment. Diagnosis is based on the presence of symptoms, but regardless of the combination of symptoms with which someone presents, it's the following criterion in the DSM-V that's necessary:
There is clear evidence that the symptoms interfere with, or reduce the quality of, social, academic, or occupational functioning.
In other words, one can be performing extremely well academically, have a high IQ, and still have ADHD. This is not to say that individuals with ADHD aren't typically poor-performing, but there's nothing precluding someone from being a high performer and still fulfilling the diagnostic criteria for ADHD:
Is attention deficit hyperactivity disorder a valid diagnosis in the presence of high IQ? Results from the MGH Longitudinal Family Studies of ADHD (Antshel et al., 2007).
Background: The aim of this study was to assess the validity of diagnosing attention deficit/hyperactivity disorder (ADHD) in high IQ children and to further characterize the clinical features associated with their ADHD.
Methods: We operationalized giftedness/high IQ as having a full scale IQ ≥120. We identified 92 children with a high IQ who did not have ADHD and 49 children with a high IQ that met diagnostic criteria for ADHD who had participated in the Massachusetts General Hospital Longitudinal Family Studies of ADHD.
Results: Of our participants with ADHD and a high IQ, the majority (n = 35) met criteria for the Combined subtype. Relative to control participants, children with ADHD and high IQ had a higher prevalence rate of familial ADHD in first‐degree relatives, repeated grades more often, had a poorer performance on the WISC‐III Block Design, had more comorbid psychopathology, and had more functional impairments across a number of domains.
Conclusions: Children with a high IQ and ADHD showed a pattern of familiality as well as cognitive, psychiatric and behavioral features consistent with the diagnosis of ADHD in children with average IQ. These data suggest that the diagnosis of ADHD is valid among high IQ children.
I note that you won't find any literature on unfulfilled potential. Instead, if you parse the above diagnostic criterion carefully, you'll see it: what precisely does the DSM-5 criterion of "reduced quality of academic functioning" mean but some inability to fulfill one's potential with regards to schooling?
ADHD as hypofrontality
Regarding hypofrontality, this has to do with the view of biological psychiatry that ADHD is mediated by dopaminergic hypofrontality. There is an attempt to map the symptoms to a particular neural substrate.
Fisher & Beckley (1998) is a little old, but characterises the general view:
That is not the end of the whole story. If the brain messengers
dopamine and norepinephrine have an impact on the frontal area of the
brain, and the hypofrontality creates the higher-level-thinking
disorder of ADD or ADHD, then wouldn't it also affect the other
thinking areas of the brain that use these same neurotransmitters?
Hypofrontality defines the lack of dopamine and norepinephrine
primarily in the frontal area of the brain. However, the effect can
also be seen in the parietal area of the brain. This defines the basis
for the anatomical differences between the two subtypes of ADD: ADHD
and ADD without hyperactivity (ADD). It has been found through both
clinical research and studies of medication that ADD without
hyperactivity is more closely related to the parietal area of the
brain and the neurotransmitter norepinephrine, while ADHD is found to
more related to the frontal area or processes and the neurotransmitter
A more recent paper, which refers to the role of D1 dopamine receptors in hypofrontality. However, it focuses more on the role on noradrenaline via a2-adrenoceptors (both dopamine and noradrenaline are catecholamines):
Neurobiology of Executive Functions: Catecholamine Influences on Prefrontal Cortical Functions (Arnsten & Li, 2005)
The prefrontal cortex guides behaviors, thoughts, and feelings using representational knowledge, i.e., working memory. These fundamental cognitive abilities subserve the so-called executive functions: the ability to inhibit inappropriate behaviors and thoughts, regulate our attention, monitor our actions, and plan and organize for the future. Neuropsychological and imaging studies indicate that these prefrontal cortex functions are weaker in patients with attention-deficit/hyperactivity disorder and contribute substantially to attention-deficit/hyperactivity disorder symptomology. Research in animals indicates that the prefrontal cortex is very sensitive to its neurochemical environment and that small changes in catecholamine modulation of prefrontal cortex cells can have profound effects on the ability of the prefrontal cortex to guide behavior. Optimal levels of norepinephrine acting at postsynaptic α-2A-adrenoceptors and dopamine acting at D1 receptors are essential to prefrontal cortex function. Blockade of norepinephrine α-2-adrenoceptors in prefrontal cortex markedly impairs prefrontal cortex function and mimics most of the symptoms of attention-deficit/hyperactivity disorder, including impulsivity and locomotor hyperactivity. Conversely, stimulation of α-2-adrenoceptors in prefrontal cortex strengthens prefrontal cortex regulation of behavior and reduces distractibility. Most effective treatments for attention-deficit/hyperactivity disorder facilitate catecholamine transmission and likely have their therapeutic actions by optimizing catecholamine actions in prefrontal cortex.
My simplistic understanding is that when there is hypofrontality, psychostimulants will improve the signal:noise ratio in the prefrontal cortex, thereby addressing the dysregulation in executive function seen in ADHD.
Cynical answer the question
ADHD is the label given to someone who is impulsive, unmotivated and bad at school and whose symptoms improve when they're given psychostimulants. This isn't a statement about whether or not ADHD does or doesn't exist, it's a statement about how its currently diagnosed and treated, hence my use of the term hypofrontality-mediated unfulfilled potential.
Antshel, K. M., Faraone, S. V., Stallone, K., Nave, A., Kaufmann, F. A., Doyle, A., ... & Biederman, J. (2007). Is attention deficit hyperactivity disorder a valid diagnosis in the presence of high IQ? Results from the MGH Longitudinal Family Studies of ADHD. Journal of Child Psychology and Psychiatry, 48(7), 687-694.
Arnsten, A. F., & Li, B. M. (2005). Neurobiology of executive functions: catecholamine influences on prefrontal cortical functions. Biological psychiatry, 57(11), 1377-1384.
Fisher, B. C., & Beckley, R. A. (1998). Attention deficit disorder: Practical coping methods. Boca Raton, FL: CRC Press.