When to seek urgent care
Chest pain or palpitations · Severe shortness of breath at rest · Confusion or altered consciousness · Sudden severe weakness or inability to move a limb · Signs of stroke (FAST: Face drooping, Arm weakness, Speech difficulty, Time to call 911)
Fatigue lasting more than 2 weeks with no clear cause · Unexplained weight loss alongside fatigue · Night sweats and fatigue together · Fatigue severe enough to interfere with daily activities · Fatigue in someone with known cancer, heart disease, or HIV
Medical causes of fatigue
| Cause | Key features | Diagnostic test |
|---|---|---|
| Anaemia (iron deficiency) | Pallor, cold intolerance, brittle nails, pica; most common in premenopausal women | FBC, ferritin, iron studies |
| Hypothyroidism | Weight gain, cold intolerance, constipation, dry skin, hair loss, slow heart rate | TSH (first-line), Free T4 |
| Diabetes | Thirst, frequent urination, blurred vision; often insidious | Fasting glucose, HbA1c |
| Heart failure | Breathlessness with exertion or lying flat, ankle swelling, orthopnoea | BNP/NT-proBNP, echocardiogram |
| Chronic kidney disease | Often asymptomatic until advanced; fatigue, oedema, uraemic symptoms | Creatinine, eGFR, urine ACR |
| Sleep apnoea | Snoring, witnessed apnoeas, morning headaches, daytime somnolence; overweight men at highest risk | Epworth score, sleep study (polysomnography) |
| Coeliac disease | GI symptoms, bloating; may be silent; B12/folate/iron deficient | Anti-tTG IgA antibodies, total IgA |
| Inflammatory conditions | Joint pain, rash, multi-system symptoms; RA, lupus, vasculitis | CRP, ESR, ANA, RF |
| Cancer | Unexplained weight loss, night sweats, lymphadenopathy, localised symptoms | FBC, LDH, imaging as indicated |
| Viral infection / post-viral | Recent illness (EBV, COVID-19, influenza); fatigue persists weeks after recovery | Monospot, EBV serology, clinical history |
| Vitamin D deficiency | Musculoskeletal pain, weakness, low mood; very common in northern latitudes | Serum 25-OH vitamin D |
| Adrenal insufficiency | Profound weakness, weight loss, hyperpigmentation, hypotension, salt craving | Short synacthen test, morning cortisol |
Mental health causes
Mental health conditions are among the most common causes of persistent fatigue — yet are frequently underdiagnosed when patients present with physical complaints.
- Depression: Fatigue is a core symptom of depression, often described as a physical heaviness or inability to initiate activity. Accompanied by low mood, loss of interest, sleep disturbance, poor concentration, and feelings of worthlessness. Fatigue from depression does not improve with rest.
- Anxiety disorders: Chronic anxiety is physically and mentally exhausting. The persistent activation of the stress response depletes energy reserves, disrupts sleep, and causes muscle tension — all contributing to fatigue.
- Burnout: Occupational exhaustion characterised by emotional exhaustion, depersonalisation, and reduced sense of accomplishment. Becoming increasingly recognised as a distinct clinical entity. Requires addressing work-life balance and often psychological support.
- Grief and bereavement: Profound physical fatigue is a normal and common feature of grief, which engages the same neurobiological stress pathways as clinical depression.
Lifestyle and sleep causes
😴 Poor Sleep Quality
- Insomnia affects 30% of adults
- Fragmented sleep equally harmful as short sleep
- CBT-I is first-line treatment
- Screen exposure delays melatonin release
🍔 Poor Nutrition
- Ultra-processed food drives fatigue
- Skipping meals causes glucose dips
- Inadequate protein reduces energy
- Dehydration (even mild) causes tiredness
🛋️ Sedentary Lifestyle
- Inactivity paradoxically worsens fatigue
- Exercise increases mitochondrial density
- Even 20 min walks show energy benefit
- Start low, build gradually
🍷 Alcohol & Substances
- Alcohol fragments sleep architecture
- Caffeine excess causes rebound fatigue
- Cannabis disrupts REM sleep
- Medication side effects common cause
ME/CFS: Myalgic Encephalomyelitis / Chronic Fatigue Syndrome
ME/CFS is a complex, debilitating, multi-system condition estimated to affect 0.9% of the global population — approximately 17–24 million Americans. It is not simply "feeling tired" — it is a disabling illness that severely impacts quality of life and, for many patients, makes full-time work or education impossible.
Diagnostic criteria (IOM 2015)
All three core features must be present for at least 6 months:
- Profound fatigue not explained by other conditions, substantially reducing ability to engage in pre-illness activities, not improved by rest
- Post-exertional malaise (PEM) — worsening of symptoms after physical or cognitive activity, often delayed 12–48 hours; the hallmark feature that distinguishes ME/CFS
- Unrefreshing sleep — sleep does not restore energy regardless of duration
Plus at least one of: cognitive impairment ("brain fog") or orthostatic intolerance (worsening on standing upright).
The instinct to "push through" fatigue — which is helpful for deconditioning — is actively harmful in ME/CFS. Exceeding the individual's energy envelope triggers post-exertional malaise and can cause prolonged worsening. The current management approach centres on pacing — staying within the energy envelope — not graded exercise therapy.
Long COVID fatigue
Long COVID — defined as symptoms persisting more than 12 weeks after acute SARS-CoV-2 infection — affects an estimated 10–30% of people who had COVID-19. Fatigue is the most common symptom, reported in over 50% of Long COVID patients, and frequently overlaps clinically with ME/CFS — including post-exertional malaise.
Current evidence suggests multiple mechanisms: persistent viral reservoirs, immune dysregulation, microbiome disruption, autonomic nervous system dysfunction (POTS — postural orthostatic tachycardia syndrome being particularly common), and mitochondrial dysfunction. There is no currently approved pharmacological treatment; management is symptom-based with pacing, rehabilitation tailored to individual tolerance, and management of specific complications (POTS, sleep disturbance, cognitive symptoms).
Blood tests and investigation
A systematic approach to investigation avoids both under-investigation (missing a treatable cause) and over-investigation (causing anxiety, incidental findings, and unnecessary procedures).
| Test | What it screens for | Who needs it |
|---|---|---|
| Full blood count (FBC) | Anaemia, infection, leukaemia | All patients |
| TSH | Thyroid disease (hypo- and hyperthyroidism) | All patients |
| Fasting glucose / HbA1c | Diabetes and prediabetes | All patients |
| Ferritin + iron studies | Iron deficiency without anaemia | All, especially women |
| Vitamin B12 and folate | Deficiency causing macrocytic anaemia and neuropathy | All patients |
| Vitamin D (25-OH) | Deficiency — very common, easily treated | All patients |
| CRP and ESR | Systemic inflammation | All patients |
| Liver function tests | Liver disease | All patients |
| Creatinine and eGFR | Kidney disease | All patients |
| Calcium | Hypercalcaemia (malignancy, hyperparathyroidism) | All patients |
| ANA, anti-dsDNA, RF | Autoimmune disease | If inflammatory features present |
| Coeliac screen (anti-tTG IgA) | Coeliac disease | If GI symptoms or nutritional deficiencies |
| Short synacthen test | Adrenal insufficiency | If clinical suspicion |
| Sleep study | Obstructive sleep apnoea | If snoring, witnessed apnoeas, excessive daytime somnolence |
Evidence-based management strategies
| Strategy | Evidence | Best for |
|---|---|---|
| Treat the underlying cause | Essential first step — iron for iron deficiency, levothyroxine for hypothyroidism | All fatigue with identifiable medical cause |
| Sleep hygiene + CBT-I | Strong — CBT-I is more effective than medication for chronic insomnia | Insomnia-related fatigue |
| Graded exercise (gradual) | Moderate — effective for deconditioning, depression, and general fatigue | Not appropriate for ME/CFS or Long COVID with PEM |
| Pacing / energy management | Expert consensus — prevents PEM, central to ME/CFS management | ME/CFS, Long COVID with PEM |
| CBT for depression/anxiety | Strong — equivalent to antidepressants for mild-moderate depression | Fatigue from mental health conditions |
| CPAP for sleep apnoea | Strong — dramatically improves daytime fatigue in OSA | Confirmed obstructive sleep apnoea |
| Mediterranean diet | Moderate — associated with lower fatigue scores in multiple studies | General lifestyle fatigue |
| Mindfulness / stress reduction | Moderate — reduces fatigue in burnout, cancer-related fatigue | Burnout, chronic stress-related fatigue |
Frequently asked questions
Fatigue alone is rarely an emergency. However, seek immediate care if fatigue is accompanied by chest pain, shortness of breath, irregular heartbeat, severe headache, confusion, sudden onset extreme weakness, or signs of stroke — these may indicate a serious cardiac, neurological, or metabolic emergency.
Standard initial blood tests include: full blood count, thyroid function (TSH), blood glucose or HbA1c, liver and kidney function, inflammatory markers (CRP, ESR), iron studies (ferritin), vitamin B12 and folate, vitamin D, and calcium. Your doctor may add further tests based on clinical history and examination.
ME/CFS is a specific, debilitating condition characterised by profound fatigue lasting 6+ months that is not improved by rest, post-exertional malaise (worsening after activity — the hallmark feature), unrefreshing sleep, and cognitive difficulties or orthostatic intolerance. It is diagnosed after excluding other medical causes. General fatigue is a symptom with many possible causes that may resolve when the underlying condition is treated.
- Institute of Medicine. Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness. Washington, DC: National Academies Press; 2015.
- Deary V, et al. Chronic fatigue syndrome. BMJ. 2007;335(7635):534–540.
- Thaiss CA, et al. The microbiome and innate immunity. Nature. 2016;535:65–74.
- Michelen M, et al. Characterising Long COVID: a living systematic review. BMJ Global Health. 2021;6:e005427.
- Qaseem A, et al. Management of chronic insomnia disorder in adults. Ann Intern Med. 2016;165(2):125–133.