Fatigue: What Could Be Causing It

How Common Is Fatigue and Why Does It Matter?

Fatigue ranks among the top 10 reasons adults visit a primary-care physician. Between 5% and 10% of patients in general practice name fatigue as their chief complaint, and up to 25% report it as a secondary symptom [1]. The clinical challenge is that fatigue itself is not a diagnosis. It is a symptom shared by dozens of conditions spanning endocrine, hematologic, psychiatric, infectious, and neurologic categories.

A 2022 cross-sectional analysis published in BMJ Open found that among 571 primary-care patients presenting with fatigue lasting four weeks or longer, a definitive medical diagnosis was established in 33.4% of cases after standard workup [2]. The remaining two-thirds received either a provisional diagnosis or no specific explanation. That gap matters because fatigue drives an estimated $136 billion in lost workplace productivity annually in the United States alone [3].

The critical first question a clinician must answer: is this fatigue physiologic (caused by insufficient sleep, overexertion, or acute stress) or pathologic (driven by an underlying disease)? Physiologic fatigue resolves with rest. Pathologic fatigue does not.

Thyroid Disorders: The Most Commonly Missed Endocrine Cause

Hypothyroidism is the single most common endocrine cause of persistent fatigue and also one of the most treatable. The National Health and Nutrition Examination Survey (NHANES) found that 4.6% of the U.S. population aged 12 years and older has hypothyroidism, with the majority of cases classified as subclinical [4].

Fatigue from thyroid underactivity develops gradually. Patients often attribute months of worsening tiredness to aging or stress before a TSH test reveals the cause. The American Thyroid Association (ATA) recommends measuring serum TSH as the initial screening test [5]. A TSH above 4.5 mIU/L with a low free T4 confirms overt hypothyroidism. Subclinical hypothyroidism (elevated TSH with normal free T4) produces fatigue in roughly 10% to 15% of affected individuals, according to a 2019 meta-analysis in the European Journal of Endocrinology [6].

Dr. Elizabeth Pearce, a professor of medicine at Boston University and former ATA president, has stated: "Subclinical hypothyroidism is one of the most debated diagnoses in endocrinology because symptoms like fatigue overlap so heavily with normal aging and other conditions" [5].

Treatment with levothyroxine (typical starting dose 1.6 mcg/kg/day, adjusted every 6 to 8 weeks) resolves fatigue in the majority of overtly hypothyroid patients within 3 to 6 months.

Hyperthyroidism causes fatigue too. The mechanism differs: excess thyroid hormone accelerates metabolism, causing muscle wasting and insomnia that produce a distinctive combination of exhaustion and restlessness.

Iron Deficiency: Fatigue Before the Anemia Appears

Iron deficiency is the most prevalent nutritional deficiency worldwide, affecting an estimated 1.2 billion people according to WHO global burden data [7]. A point that primary-care guidelines now emphasize: fatigue from iron depletion begins well before hemoglobin drops below the anemia threshold.

Serum ferritin is the most sensitive single marker. A 2020 Cochrane review of 18 randomized controlled trials (N=1,170 non-anemic women with fatigue and ferritin <50 mcg/L) found that oral iron supplementation reduced subjective fatigue scores by a standardized mean difference of 0.38 (95% CI 0.13 to 0.63) compared with placebo [8]. That effect size is comparable to what many antidepressant trials report for mood symptoms.

Groups at highest risk include premenopausal women (menstrual blood loss), endurance athletes, frequent blood donors, and patients on proton pump inhibitors (which reduce iron absorption). A ferritin level below 30 mcg/L in a fatigued patient warrants a trial of iron replacement even if hemoglobin is normal.

Oral ferrous sulfate 325 mg every other day produces equivalent absorption to daily dosing with fewer GI side effects, per a 2017 study in Blood [9]. IV iron (ferric carboxymaltose, 750 mg x 2 doses) is reserved for malabsorption, intolerance, or ferritin <15 mcg/L with symptomatic anemia.

Depression and Anxiety: The Psychiatric-Medical Overlap

Depression and fatigue share a bidirectional relationship. Fatigue is one of the nine DSM-5 criteria for major depressive disorder, and it is also the most common residual symptom after otherwise successful antidepressant treatment [10]. A 2018 analysis of STAR*D trial data (N=2,876) showed that 73% of patients with depression reported significant fatigue at baseline, and 38% still reported it after achieving remission of mood symptoms [11].

This creates a diagnostic trap. Patients presenting with fatigue may have unrecognized depression; patients diagnosed with depression may have a separate medical cause of fatigue that gets attributed entirely to their mood disorder.

The PHQ-9 takes under three minutes to administer and has 88% sensitivity and 85% specificity for major depressive disorder in primary-care populations [12]. Any fatigue workup that omits a standardized mood screen is incomplete.

Generalized anxiety disorder (GAD) also produces fatigue through a different mechanism: chronic sympathetic activation and disrupted sleep architecture. The GAD-7 is the corresponding screening tool [13].

Treatment considerations for fatigue co-occurring with depression include selecting antidepressants with activating profiles. Bupropion, which acts on norepinephrine and dopamine rather than serotonin, causes less fatigue than SSRIs in head-to-head comparisons. A 2006 randomized trial published in the Journal of Clinical Psychiatry (N=345) found that patients switching from an SSRI to bupropion XL 300 mg reported a 40% improvement in fatigue scores over 8 weeks [14].

Obstructive Sleep Apnea: The Epidemic Hiding in Plain Sight

Obstructive sleep apnea (OSA) is dramatically underdiagnosed. A 2019 prevalence analysis published in The Lancet Respiratory Medicine estimated that 936 million adults aged 30 to 69 worldwide have mild to severe OSA, and 425 million have moderate to severe disease [15]. In the United States, an estimated 80% of moderate-to-severe cases remain undiagnosed [16].

The cardinal symptom triad is snoring, witnessed apneas, and excessive daytime sleepiness. But many patients deny all three and present only with fatigue, morning headaches, or difficulty concentrating.

The STOP-BANG questionnaire (Snoring, Tiredness, Observed apneas, Pressure [blood pressure], BMI >35, Age >50, Neck circumference >40 cm, Gender male) scores 0 to 8. A score of 5 or higher has 83.6% sensitivity for moderate-to-severe OSA [17].

Home sleep apnea testing (HSAT) has largely replaced in-lab polysomnography for uncomplicated cases. CPAP therapy at adequate pressure (auto-titrating devices are most common) reduces the Epworth Sleepiness Scale score by an average of 2.9 points compared with sham CPAP in the APPLES trial [18].

Dr. Sanjay Patel, professor of medicine at the University of Pittsburgh, noted in a 2021 ATS interview: "We should be screening for sleep apnea as routinely as we screen for hypertension, given the cardiovascular and metabolic consequences of untreated disease."

Diabetes and Prediabetes: Metabolic Fatigue

Type 2 diabetes and prediabetes together affect over 96 million American adults, per 2022 CDC National Diabetes Statistics [19]. Fatigue is reported by 40% to 60% of patients with type 2 diabetes, and it often precedes the diagnosis by months or years.

The mechanism is straightforward: impaired glucose uptake means cells cannot generate adequate ATP despite elevated blood glucose. Glycemic variability (spikes and crashes) compounds the problem. A 2017 study using continuous glucose monitoring in 60 patients with type 2 diabetes found that fatigue severity correlated more strongly with glucose variability (MAGE index) than with HbA1c [20].

Screening starts with fasting plasma glucose or HbA1c. An HbA1c of 5.7% to 6.4% indicates prediabetes; 6.5% or higher indicates diabetes [21]. A fatigued patient with a fasting glucose of 110 mg/dL deserves a full oral glucose tolerance test, not reassurance.

Metformin, the first-line medication for type 2 diabetes, has itself been associated with fatigue in some patients, often through vitamin B12 depletion. The ADA Standards of Care recommend periodic B12 monitoring in patients on long-term metformin [21].

Vitamin D Deficiency: Genuine Contributor or Overdiagnosed?

Vitamin D deficiency (25-hydroxyvitamin D <20 ng/mL) affects an estimated 41.6% of U.S. adults, with higher rates in Black (82.1%) and Hispanic (69.2%) adults [22]. The question is whether low vitamin D actually causes fatigue or simply coexists with it.

A 2019 systematic review and meta-analysis of 7 RCTs (N=3,084) published in Medicine found that vitamin D supplementation improved fatigue scores in vitamin D-deficient individuals compared with placebo (standardized mean difference 0.37, 95% CI 0.07 to 0.67), but the effect was absent in vitamin D-sufficient populations [23]. The clinical takeaway: test before you treat, and only expect benefit in truly deficient patients.

Repletion protocols vary. The Endocrine Society guideline recommends 50,000 IU ergocalciferol weekly for 8 weeks followed by maintenance of 1,500 to 2,000 IU daily for patients with levels below 20 ng/mL [24].

Medications That Cause Fatigue

Drug-induced fatigue is among the most underrecognized causes because patients and clinicians both tend to attribute the symptom to the condition being treated rather than to the treatment itself.

Common offenders include beta-blockers (especially non-selective agents like propranolol), first-generation antihistamines, benzodiazepines, opioids, SSRIs (particularly paroxetine), statins, and anticonvulsants [25]. The timing pattern is the diagnostic clue: fatigue beginning within days to weeks of a new medication or dose increase.

A practical approach is the "drug holiday" for non-essential medications, conducted in coordination with the prescriber. For essential medications, dose reduction or switching within the same class often resolves the issue. Beta-blocker fatigue, for example, frequently improves when switching from propranolol to nebivolol, which has vasodilating properties and less central nervous system penetration.

Antihistamine-related fatigue is worth special mention because patients often do not consider over-the-counter allergy medications as "real" drugs. Diphenhydramine (Benadryl) crosses the blood-brain barrier and causes sedation that persists well beyond the perceived sleep benefit. Second-generation antihistamines (cetirizine, loratadine, fexofenadine) are significantly less sedating [25].

Chronic Fatigue Syndrome (ME/CFS): When the Workup Is Negative

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a diagnosis of exclusion that requires fatigue lasting 6 months or longer, substantial reduction in pre-illness activity levels, post-exertional malaise, and unrefreshing sleep [26]. The 2015 IOM (now NAM) report estimated that 836,000 to 2.5 million Americans have ME/CFS, with the majority undiagnosed [26].

This is not a wastebasket diagnosis. Post-exertional malaise (PEM), the hallmark feature, distinguishes ME/CFS from deconditioning and depression. PEM manifests as a disproportionate worsening of symptoms 12 to 72 hours after physical or cognitive exertion that would not have caused symptoms before illness onset.

No FDA-approved treatments exist specifically for ME/CFS. Management focuses on activity pacing (staying within the "energy envelope"), treating comorbid sleep disorders, and low-dose medications for specific symptoms. Graded exercise therapy, once widely recommended, was downgraded by NICE in 2021 after reanalysis of the PACE trial (N=641) raised concerns about bias and the durability of reported benefits [27].

The Diagnostic Workup: A Practical Checklist

A cost-effective initial fatigue workup includes the following laboratory tests: CBC with differential, comprehensive metabolic panel, TSH, free T4, ferritin, fasting glucose or HbA1c, 25-hydroxyvitamin D, ESR or CRP, and urinalysis [28]. This panel costs $80 to $200 at most commercial laboratories and identifies the cause in approximately one-third of cases.

Second-tier testing, reserved for cases where the initial workup is unrevealing, includes cortisol (morning or ACTH stimulation test), ANA, hepatitis B and C serologies, HIV, celiac panel (tTG-IgA), and testosterone (in men with concurrent low libido or erectile dysfunction) [28].

Red flags warranting urgent or expedited evaluation:

• Unintentional weight loss exceeding 5% of body weight over 6 months
• Drenching night sweats
• New or enlarging lymph nodes
• Fever lasting more than 3 weeks
• Progressive neurological symptoms
• Fatigue with dyspnea at rest or new-onset heart failure symptoms

Any of these findings shifts the workup from outpatient to urgent, with imaging and potentially biopsy as next steps.

When Hormones Are the Missing Piece

Adrenal insufficiency, both primary (Addison disease) and secondary (from chronic corticosteroid use or pituitary pathology), presents with fatigue as the predominant symptom in over 90% of cases [29]. The disease is rare (prevalence approximately 100 per million for primary adrenal insufficiency), but the consequences of missing it are severe.

Morning cortisol below 3 mcg/dL is diagnostic. Values between 3 and 15 mcg/dL require an ACTH stimulation test for confirmation [29]. Any patient on chronic prednisone (more than 7.5 mg daily for 3 or more weeks) who reports worsening fatigue after dose reduction should be evaluated for secondary adrenal insufficiency.

Testosterone deficiency in men presents with fatigue alongside decreased libido, reduced muscle mass, and increased body fat. The Endocrine Society Clinical Practice Guideline recommends testing total testosterone (two morning samples) in men with consistent symptoms, with a threshold of 264 to 300 ng/dL for defining deficiency [30].

For women in perimenopause and menopause, declining estradiol contributes to fatigue both directly and through sleep disruption from vasomotor symptoms. Hormone therapy in symptomatic women within 10 years of menopause onset improves fatigue, sleep quality, and overall well-being, as demonstrated in the Kronos Early Estrogen Prevention Study (KEEPS) [31].

Building a Treatment Plan Based on Root Cause

Treating fatigue without a diagnosis is like treating a fever without knowing the infection. The algorithm is: identify the cause, treat the cause, reassess at a defined interval.

For thyroid disease: levothyroxine with TSH recheck at 6 to 8 weeks. For iron deficiency: oral or IV iron with ferritin recheck at 8 to 12 weeks. For depression: antidepressant therapy (consider bupropion if fatigue is prominent) with PHQ-9 reassessment at 4 to 6 weeks. For sleep apnea: CPAP with adherence review and Epworth Sleepiness Scale at 30 days. For diabetes: glycemic optimization targeting glucose variability reduction. For vitamin D deficiency: repletion protocol with level recheck at 3 months.

Patients whose fatigue persists despite treatment of an identified cause should be re-evaluated for a second concurrent diagnosis. The co-occurrence rate of thyroid disease plus iron deficiency, or depression plus sleep apnea, is high enough that a single explanation may be incomplete.

A first-line fatigue panel (CBC, CMP, TSH, ferritin, HbA1c, vitamin D, CRP) identifies or excludes the most common medical causes for under $200 and should be ordered for any patient reporting fatigue lasting more than four weeks [28].