Someone books an evaluation because they cannot concentrate, cannot finish things, lose their train of thought mid-sentence, and have started making errors at work they would not have made two years ago. They have read about ADHD and it fits.
Sometimes it is ADHD, and the evaluation confirms what they already suspected. Sometimes the description fits ADHD symptoms perfectly and still is not ADHD, because inattention and executive dysfunction are not specific findings. They are the cognitive equivalent of a fever: real, measurable, and produced by a long list of different things.
Sorting that out is most of what an evaluation is for.
The Question That Does the Work
ADHD is a neurodevelopmental condition. The diagnostic criteria require that symptoms were present in childhood and that they are not better explained by another condition.
Those two requirements are the entire differential in compressed form, and they are also the two most commonly skipped.
If attention was fine through school, through early jobs, through the first thirty years of adult life, and then deteriorated at 38, that trajectory is not ADHD. Something changed, and finding out what changed is the actual clinical task. ADHD does not switch on in adulthood, though it very often becomes visible in adulthood when demands rise past the point compensation can cover. Those two situations look similar in the first ten minutes of a conversation and are completely different problems.
Obstructive Sleep Apnea
This is the one I most want people to know about, because it is common, treatable, and routinely missed.
A 2019 analysis in The Lancet Respiratory Medicine estimated that 936 million adults aged 30 to 69 worldwide have mild to severe obstructive sleep apnea, with 425 million in the moderate to severe range.1 A large share have never been diagnosed.
Untreated sleep apnea fragments sleep and produces intermittent drops in oxygen, and the cognitive consequences are well documented: deficits in attention, executive function, memory, and processing speed. Someone in that state is inattentive, forgetful, irritable, and cognitively slowed. They will describe it as brain fog or as never being able to focus, and every word of that description will be accurate.
Two things make this especially easy to miss. The classic picture of loud snoring in an overweight middle-aged man leaves out a great many people who have it, including women, thin people, and anyone with craniofacial anatomy that narrows the airway. And stimulant medication will partially mask the daytime sleepiness, which can make an ADHD trial look like a partial success while the underlying apnea goes on damaging cardiovascular health untreated.
I want to be accurate about the state of the evidence here. The literature specifically examining whether treating sleep apnea improves ADHD-type symptoms in adults is thin, consisting largely of case reports and small studies rather than trials. What is well established is that untreated apnea impairs cognition and that the condition is dramatically underdiagnosed. That is enough to justify screening in an ADHD evaluation.
Depression
Depression produces genuine cognitive impairment, not just the subjective sense of it.
A 2019 systematic review and meta-analysis in The Lancet Psychiatry pooled 252 studies comparing nearly 12,000 people in remission from a major depressive episode against more than 8,500 healthy controls.2 Significant deficits were still present in 55 of the 75 cognitive variables examined, spanning processing speed, visual selective attention, working memory, verbal learning, and executive function. Most of those deficits were small or medium in size, though several long-term memory measures showed large ones.
The phrase worth holding onto there is "in remission." These were people whose depressive episode had resolved, and the cognitive impairment had not resolved with it.
That has a practical consequence. Someone whose mood has responded to an antidepressant but whose concentration has not is a common presentation, and it does not by itself mean the residual problem is ADHD. It may be residual cognitive impairment from the depression. Untangling those requires the developmental history, not a guess.
So a depressed adult may present with exactly the complaint that brings people to an ADHD evaluation, and the impairment is real rather than imagined. The distinguishing questions are about trajectory and company. Did the concentration problems arrive with the low mood, the anhedonia, the sleep and appetite changes, or did they predate all of it by twenty years?
Chronic Insufficient Sleep
Not a disorder, and worth naming separately because it is so common it becomes invisible.
An adult sleeping five and a half hours a night for years will have impaired attention, working memory, and emotional regulation. This is not a subtle effect and it does not require any pathology to explain it. It also compounds with everything else on this list, and it is one of the more common reasons a medication that was working seems to stop.
Thyroid Dysfunction and Iron Deficiency
Both belong in the workup because both are common, both produce fatigue and cognitive complaints, and both are inexpensive to check.
Hypothyroidism produces cognitive slowing, fatigue, and low mood. Iron deficiency, including iron deficiency without frank anemia, is common in menstruating women and produces fatigue and difficulty concentrating.
Neither of these is usually the whole answer in an adult presenting with lifelong inattention. But finding and correcting one costs very little, and leaving one uncorrected while escalating stimulant doses against it is a bad trade.
Anxiety
Anxiety consumes working memory. Someone running a continuous background process of worry has less cognitive capacity available for the task in front of them, and the experience of that is indistinguishable from inattention.
The relationship here is genuinely bidirectional and is one of the harder calls in this differential, partly because anxiety disorders and ADHD co-occur at high rates. Anxiety and depression are among the conditions treated alongside ADHD for exactly that reason.
The Mistake Runs Both Directions
Here is where the framing of this post could mislead, so I want to correct it explicitly.
The point is not that these conditions are alternatives to ADHD, competing for one slot. In practice they coexist with it constantly. Adults with ADHD have elevated rates of sleep disorders, depression, and anxiety, and identifying one of those does not resolve the ADHD question.
The failure mode I see most often is not overdiagnosis. It is a provider finding one plausible explanation, stopping there, and treating that alone for years while the ADHD goes unaddressed. Someone gets an antidepressant, gets partially better, and never gets asked what their attention was doing in fourth grade.
Both errors are real. Diagnosing ADHD without ruling anything out, and diagnosing depression or anxiety and never asking about the developmental history. The second one is more common and does more damage, because it can continue unchallenged for a decade. The same pattern shows up when reactive mood shifts get read as bipolar disorder, and when compensatory checking gets read as OCD.
What a Careful Evaluation Should Include
A developmental history that actually goes back to childhood, including school performance, report card comments, and how things went before anyone was managing anything.
A sleep history that asks about snoring, witnessed apneas, morning headaches, and unrefreshing sleep, not just about hours slept.
A screen for depression and anxiety with attention to sequence, meaning which came first.
Basic labs, including thyroid function and iron studies where indicated, or confirmation that they were checked recently.
And a willingness to say that the picture is not clear yet and something needs to be ruled out first. That answer is less satisfying than a diagnosis at the first visit, and it is frequently the correct one.
If You Are Considering an Evaluation
None of this is a reason to talk yourself out of being assessed. If your concentration has changed, something is producing that, and it is worth identifying.
What it is a reason for is choosing an evaluation that will actually consider the alternatives rather than confirming what you arrived with. A provider who reaches ADHD after asking about your sleep, your mood, your childhood, and your labs has told you something. A provider who reaches it in fifteen minutes from a symptom checklist has told you considerably less.
The wider picture of how the diagnosis is made, and what treatment does once it is, is covered in the guide to adult ADHD.
References
1. Benjafield AV, Ayas NT, Eastwood PR, et al. Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis. The Lancet Respiratory Medicine. 2019;7(8):687 to 698. doi:10.1016/S2213-2600(19)30198-5
2. Semkovska M, Quinlivan L, O'Grady T, et al. Cognitive function following a major depressive episode: a systematic review and meta-analysis. The Lancet Psychiatry. 2019;6(10):851 to 861. doi:10.1016/S2215-0366(19)30291-3
Depression is one of the most common look-alikes and companions. ADHD and depression in adults covers how the two get told apart.
Clarity ADHD is a telehealth practice focused on adult ADHD care in Washington State offering adult ADHD evaluation and medication management, including care for co-occurring depression and anxiety. Now accepting new patients, and you can start with a free 15-minute consultation to see if it is a fit.
Book a Free 15-Minute ConsultationThe content on this website is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.