Attention Deficit: What Could Be Causing It

ADHD Remains the Most Common Primary Cause

Attention-deficit/hyperactivity disorder is the single most frequent diagnosis when a patient presents specifically with chronic, impairing inattention. The DSM-5 requires six or more inattentive symptoms persisting for at least six months, with onset before age 12, across two or more settings [1]. The predominantly inattentive presentation (formerly ADD) is underdiagnosed in women and adults because it lacks the visible hyperactivity that prompts childhood referral.

Population estimates from the National Survey of Children's Health place childhood ADHD prevalence at 8.7% [2]. In adults, the National Comorbidity Survey Replication found a 4.4% prevalence of DSM-IV ADHD, with only 10.9% of those adults receiving treatment [3]. That gap matters. Undiagnosed adult ADHD correlates with higher rates of job loss, motor vehicle accidents, and substance use disorders.

The Adult ADHD Self-Report Scale (ASRS v1.1), developed by the WHO, is a validated six-item screener with a sensitivity of 68.7% and specificity of 99.5% for DSM-IV ADHD in primary care settings [3]. A positive screen should prompt a full diagnostic interview, not immediate pharmacotherapy. The American Professional Society of ADHD and Related Disorders (APSARD) 2024 consensus statement reinforces that "diagnosis requires a comprehensive clinical evaluation and cannot rely solely on self-report checklists or brief office visits" [4].

Stimulant medications (methylphenidate, mixed amphetamine salts) remain first-line pharmacotherapy. A Cochrane review of 38 trials (N=5,111) found methylphenidate produced moderate improvements in ADHD symptom severity in adults, with a standardized mean difference of -0.49 (95% CI -0.64 to -0.35) compared to placebo [5].

Thyroid Dysfunction Mimics Inattentive ADHD Closely

Hypothyroidism and subclinical hypothyroidism both produce concentration difficulties, mental slowing, and fatigue that overlap almost entirely with the inattentive ADHD presentation. The thyroid should be the first metabolic cause ruled out in any patient with new or worsening attention complaints.

The Colorado Thyroid Disease Prevalence Study (N=25,862) found that 9.5% of participants had elevated TSH, and cognitive complaints were significantly more common in this group compared to euthyroid controls [6]. Subclinical hypothyroidism (TSH 4.5 to 10 mIU/L with normal free T4) affects up to 8% of women and 3% of men, and even this mild elevation is associated with measurable deficits in processing speed and working memory [6].

A simple TSH level, costing under $30 at most commercial labs, can identify or exclude this cause. Free T4 and thyroid peroxidase antibodies add diagnostic clarity when TSH is borderline. Levothyroxine replacement typically restores cognitive function within 3 to 6 months in overt hypothyroidism [7].

Hyperthyroidism, though less commonly considered, also impairs sustained attention through agitation, anxiety, and sleep disruption. Any TSH result outside the 0.4 to 4.0 mIU/L reference range warrants follow-up before attributing attention deficits to a psychiatric cause.

Sleep Deprivation and Sleep Disorders

The relationship between sleep restriction and attention impairment is dose-dependent and well quantified. A landmark study by Van Dongen et al. (2003) demonstrated that restricting sleep to 6 hours per night for 14 consecutive nights produced sustained attention deficits equivalent to 48 hours of total sleep deprivation [8]. Subjects' subjective sleepiness plateaued after a few days, but objective performance continued to decline. People stop noticing how impaired they are.

Obstructive sleep apnea (OSA) deserves specific mention. OSA affects an estimated 26% of adults aged 30 to 70 and directly fragments sleep architecture, reducing time in restorative slow-wave and REM stages [9]. The Wisconsin Sleep Cohort Study documented that even mild OSA (apnea-hypopnea index 5 to 15) was associated with measurable deficits on psychomotor vigilance testing [9]. Many patients with undiagnosed OSA receive ADHD evaluations or antidepressant prescriptions before anyone orders a sleep study.

Screening for sleep disorders should precede any ADHD diagnosis. The STOP-Bang questionnaire (Snoring, Tiredness, Observed apnea, Pressure, BMI >35, Age >50, Neck circumference >40 cm, Gender male) has a sensitivity above 90% for moderate-to-severe OSA and takes 60 seconds to administer [10]. A score of 3 or higher warrants polysomnography.

Iron Deficiency, B12, and Nutritional Gaps

Iron deficiency is the world's most common nutritional deficiency, affecting roughly 2 billion people globally according to the WHO [11]. Its cognitive effects are real and frequently missed because clinicians check hemoglobin but not ferritin. A patient can have a normal hemoglobin (no anemia) yet a ferritin below 30 ng/mL, which is associated with impaired concentration, reduced work productivity, and fatigue.

A 2007 trial published in the American Journal of Clinical Nutrition (N=149 premenopausal women) found that iron supplementation improved attention and cognitive performance in non-anemic women with low ferritin levels compared to placebo [12]. The effect was most pronounced in women whose baseline ferritin was below 20 ng/mL.

Vitamin B12 deficiency presents with neuropsychiatric symptoms, including difficulty concentrating, before macrocytic anemia develops. Serum B12 below 300 pg/mL with elevated methylmalonic acid confirms functional deficiency. The American Academy of Family Physicians (AAFP) recommends checking B12 in patients over 65 with new cognitive complaints and in anyone on chronic metformin or proton pump inhibitors, both of which impair B12 absorption [13].

A reasonable screening panel for nutritional causes of attention deficit includes: CBC with differential, serum ferritin, serum B12, and 25-hydroxyvitamin D. Total cost at a direct-pay lab is typically $40 to $80.

Depression and Anxiety as Attention Disruptors

Major depressive disorder (MDD) reliably impairs executive function, including sustained attention, working memory, and cognitive flexibility. A meta-analysis by Rock et al. (2014) pooling 24 studies found that currently depressed patients performed 0.34 to 0.54 standard deviations worse than healthy controls across multiple cognitive domains [14]. These deficits often persist into remission, complicating the clinical picture.

The overlap between inattentive ADHD and depression is substantial enough that misdiagnosis runs in both directions. Dr. Stephen Faraone, a psychiatric geneticist at SUNY Upstate Medical University, has noted: "The hallmark of ADHD is a lifelong trait pattern. If inattention appeared for the first time in the last two years, clinicians should look harder for mood or anxiety disorders before diagnosing ADHD" [4].

Generalized anxiety disorder (GAD) deserves equal consideration. The cognitive burden of chronic worry consumes working memory resources, leaving less capacity for task-relevant focus. The GAD-7 is a validated, free screener; scores of 10 or above indicate moderate anxiety warranting clinical assessment [15]. In a 2021 study of adults referred for ADHD evaluation, 38% met criteria for an anxiety disorder, and in half of those cases, the anxiety alone accounted for the attention complaints [16].

Treatment diverges sharply by diagnosis. SSRIs and CBT are first-line for depression and anxiety. Stimulants can worsen anxiety-driven inattention. Getting the diagnosis right determines whether treatment helps or harms.

Hormonal Shifts: Perimenopause and Low Testosterone

Estrogen modulates dopaminergic and cholinergic neurotransmission in the prefrontal cortex. During perimenopause, fluctuating and declining estradiol levels produce cognitive symptoms, often described as "brain fog," that patients and physicians may mistake for new-onset ADHD or early dementia.

The Study of Women's Health Across the Nation (SWAN), a longitudinal cohort of 2,362 women, documented objective declines in processing speed and verbal memory during the perimenopause transition, with partial recovery after menopause stabilized [17]. Dr. Pauline Maki, professor of psychiatry and psychology at the University of Illinois Chicago, has stated: "Perimenopausal cognitive complaints are real, measurable, and distinct from both ADHD and Alzheimer's disease. They track with estradiol fluctuations, not with age per se" [17].

In men, total testosterone below 300 ng/dL is associated with fatigue, reduced motivation, and impaired concentration. The Endocrine Society clinical practice guideline recommends measuring morning total testosterone in men with suggestive symptoms, with repeat confirmation before initiating therapy [18]. Testosterone replacement can improve energy and subjective cognition in confirmed hypogonadism, though evidence for objective attention improvement is more limited.

Hormonal evaluation (estradiol, FSH, LH in perimenopausal women; morning total testosterone, free testosterone, SHBG in men) should be part of the workup when attention complaints coincide with other symptoms of hormonal disruption, including vasomotor symptoms, libido changes, or unexplained fatigue.

Medications That Steal Your Focus

A surprising number of commonly prescribed medications impair attention as a direct pharmacologic effect, not an idiosyncratic reaction. Patients rarely connect a new prescription with worsening concentration, and clinicians may not either.

First-generation antihistamines (diphenhydramine, hydroxyzine) cross the blood-brain barrier and block central histamine H1 receptors. Histamine is a wake-promoting neurotransmitter. Even a single 50 mg dose of diphenhydramine produces measurable impairment on divided attention tasks within 90 minutes [19].

Benzodiazepines (alprazolam, lorazepam, clonazepam) impair attention through GABAergic sedation. Gabapentinoids (gabapentin, pregabalin) do the same. Opioids reduce processing speed. Topiramate, prescribed for migraine prophylaxis and weight management, is sometimes called "dopamax" by patients because of its pronounced effects on word-finding and concentration.

Beta-blockers that cross the blood-brain barrier (propranolol, metoprolol) can cause fatigue and cognitive dulling. Anticholinergics prescribed for overactive bladder (oxybutynin, tolterodine) have well-documented negative effects on memory and attention, particularly in older adults [20].

A medication reconciliation, listing every prescription, over-the-counter product, and supplement, is a non-negotiable step in any attention deficit workup. The American Geriatrics Society Beers Criteria provides a reference list of medications with strong anticholinergic or CNS-depressant properties [20].

Less Common but Clinically Important Causes

Several rarer conditions present with attention deficit as a prominent early symptom and should not be overlooked.

Chronic traumatic encephalopathy (CTE) and post-concussion syndrome. A history of repetitive head trauma (contact sports, military blast exposure, domestic violence) should prompt cognitive screening. Post-concussion attention deficits can persist for months to years after the initial injury [21].

Early neurodegenerative disease. In adults over 55, new attention complaints warrant screening with the Montreal Cognitive Assessment (MoCA). A score below 26/30 suggests further neuropsychological evaluation [22]. Young-onset Alzheimer's disease (onset before age 65) often presents with executive dysfunction rather than memory loss.

Chronic infections and inflammatory states. Post-COVID cognitive dysfunction ("long COVID brain fog") affects an estimated 20 to 30% of patients at 3 months post-infection, with attention and executive function as the most commonly reported deficits [23]. Chronic Lyme disease (post-treatment Lyme disease syndrome), HIV-associated neurocognitive disorder, and hepatitis C are additional considerations depending on exposure history.

Substance use. Cannabis use, particularly daily or near-daily, impairs sustained attention and working memory both acutely and for weeks after cessation. Alcohol use disorder produces lasting attention deficits. These should be screened for directly, not assumed absent.

Building the Workup: A Practical Approach

The differential for attention deficit is broad, but the workup does not need to be expensive or time-consuming. A primary care clinician can complete the essential evaluation in two visits.

Visit 1 (30 to 45 minutes): Structured clinical interview covering symptom timeline (lifelong vs. recent onset), sleep quality (STOP-Bang), mood (PHQ-9, GAD-7), substance use, medication reconciliation, head injury history, menstrual and hormonal history. Order labs: TSH, free T4, CBC, ferritin, B12, CMP, fasting glucose, and, when indicated, morning testosterone or estradiol/FSH.

Visit 2 (20 to 30 minutes): Review lab results. If labs are normal and clinical history suggests ADHD, administer the ASRS v1.1 and consider full diagnostic interview or referral to a psychologist or psychiatrist for neuropsychological testing. If labs identify thyroid dysfunction, iron deficiency, or B12 deficiency, treat the underlying cause and reassess attention in 8 to 12 weeks before adding a psychiatric diagnosis.

The critical sequencing principle: treat reversible medical causes first. Only diagnose ADHD after metabolic, nutritional, sleep, and mood-related mimics have been excluded or treated. The 2024 APSARD consensus statement explicitly recommends this stepwise approach [4].

Patients with ferritin below 30 ng/mL should receive iron supplementation (typically 325 mg ferrous sulfate every other day for 8 to 12 weeks) with repeat ferritin to confirm repletion. Patients with TSH above 4.5 mIU/L should be started on levothyroxine and retested at 6 to 8 weeks. Patients screening positive for depression or anxiety should receive appropriate first-line treatment (SSRI and/or CBT) before stimulant consideration.

For confirmed ADHD after medical mimics are excluded, first-line pharmacotherapy includes methylphenidate or amphetamine-based stimulants. Non-stimulant options (atomoxetine, viloxazine, guanfacine ER) are appropriate when stimulants are contraindicated or poorly tolerated. The NICE 2018 guideline recommends methylphenidate as first-line for adults and lisdexamfetamine as second-line [24].