Causes of Fatigue By Organ System

Associated Symptoms

• Dyspnea on exertion
• Orthopnea
• Paroxysmal nocturnal dyspnea
• Chest discomfort
• Palpitations
• Peripheral edema
• Reduced exercise tolerance

Clinical Picture

This is low cardiac output fatigue. Patients describe:

"I get tired walking across the room."

Progressive exertional intolerance. Fatigue proportional to activity.

Often accompanied by: Tachycardia, S3 gallop, Hypotension or pulse pressure changes.

Think: Heart failure, Valvular disease, Arrhythmia, Ischemic disease.

Associated Symptoms

• Dyspnea
• Chronic cough
• Wheezing
• Snoring or witnessed apneas
• Morning headaches
• Cyanosis (late)

Clinical Picture

This is hypoxic or sleep-fragmentation fatigue.

Patients may wake unrefreshed, require daytime naps, or have exertional shortness of breath.

Classic in: COPD, Interstitial lung disease, Obstructive sleep apnea.

Fatigue improves with oxygenation correction or sleep normalization.

Associated Symptoms

• Headache
• Cognitive slowing ("brain fog")
• Memory difficulty
• Sleep disturbance
• Focal weakness
• Sensory changes
• Ataxia
• Tremor

Clinical Picture

Fatigue here is often described as mental exhaustion rather than pure physical weakness. Patients may say:

"My brain feels tired."
"I can't think clearly."
"After mental effort I'm wiped out."

Consider: Neurodegenerative disease, Demyelinating disorders, Post-concussive states, Central sleep disorders.

Fatigue often worsens with cognitive exertion.

Associated Symptoms

• Anhedonia
• Insomnia or hypersomnia
• Appetite change
• Reduced concentration
• Anxiety
• Low motivation

Clinical Picture

This is often motivational fatigue and emotional exhaustion. Patients may say:

"I just don't feel like doing anything."
"Everything feels like effort."

Seen in: Major depressive disorder, Anxiety disorders, Chronic stress states.

Associated Symptoms

• Loud snoring
• Restless legs
• Frequent awakenings
• Morning headaches
• Daytime somnolence

Clinical Picture

Patients sleep adequate hours but awaken unrefreshed.

Think: Obstructive sleep apnea, Periodic limb movement disorder, Insomnia disorder.

Associated Symptoms

• Weight gain or loss
• Cold or heat intolerance
• Hair changes
• Constipation or diarrhea
• Polyuria / polydipsia
• Libido changes

Clinical Picture

Hypothyroid pattern: Slowed mentation, Cold intolerance, Dry skin, Bradycardia.

Hyperthyroid pattern: Fatigue + nervous energy, Weight loss, Tremor, Tachycardia.

Adrenal insufficiency: Orthostatic symptoms, Hyperpigmentation, Salt craving.

This is often metabolic inefficiency fatigue.

Associated Symptoms

• Myalgias
• Joint pain
• Morning stiffness
• Muscle weakness
• Reduced grip strength

Clinical Picture

Fatigue here may be activity-induced muscle exhaustion or inflammatory stiffness improving with movement.

Think: Polymyalgia rheumatica, Myopathies, Fibromyalgia.

Patients often distinguish between "weakness" and "tiredness."

Associated Symptoms

• Anorexia
• Nausea
• Early satiety
• Jaundice
• Abdominal pain
• Dark urine

Clinical Picture

Fatigue is often gradual and associated with malaise.

In liver disease: Sleep–wake reversal, Pruritus, Muscle wasting.

Often biochemical abnormalities precede overt symptoms.

Associated Symptoms

• Fever or low-grade temperature elevation
• Night sweats
• Weight loss
• Arthralgias
• Lymphadenopathy
• Rash

Clinical Picture

This is cytokine-mediated fatigue. Often described as:

"Flu-like exhaustion"

Profound malaise. Seen in: Chronic infections, Autoimmune disease, Malignancy.

Fatigue may be disproportionate to exam findings.

Associated Symptoms

• Pallor
• Lightheadedness
• Tachycardia
• Exertional dyspnea
• Pica (iron deficiency)
• Bruising (if marrow disease)

Clinical Picture

This is oxygen delivery failure fatigue. Rapid exhaustion with minimal effort.

Classic in: Anemia (iron, B12, chronic disease), Bone marrow disorders.

Objective findings often precede dramatic symptoms.