Blood disorders or cancers

Anemia

Decreased exercise tolerance with shortness of breath during activity greater than expected for the activity

Sometimes paleness

CBC

Cancers (such as digestive tract cancer, lung cancer, leukemia, lymphoma, multiple myeloma)

Widespread lymph node swelling, weight loss, and/or night sweats

With multiple myeloma, low back or other bone pain, often severe at night

CBC

Sometimes other tests, such as endoscopy or imaging tests

Infections

Chronic hepatitis

Sometimes jaundice, loss of appetite, and fluid in the abdomen

Blood tests to determine how well the liver is functioning and to identify the hepatitis virus

Sometimes liver biopsy

COVID-19

Fever

Sometimes cough and respiratory symptoms

Sometimes loss of taste and smell

Test for SARS-CoV-2

Cytomegalovirus infection

Enlarged spleen and lymph nodes, fever, and night sweats

Sometimes blood tests for antibodies to cytomegalovirus

Heart valve infection (endocarditis)

Fever and night sweats

Sometimes joint pains

Usually in people who have heart murmurs or inject drugs intravenously

Cultures of blood samples and echocardiography

HIV/AIDS

Fever, night sweats, and frequent infections

Sometimes difficulty breathing, cough, diarrhea, and/or rash

HIV blood test

Mononucleosis

Recent sore throat and lymph node swelling

Mononucleosis blood test

Other infections (for example, fungal pneumonias such as histoplasmosis, parasitic infections, or tuberculosis)

Fever, night sweats, and weight loss

Sometimes cough, difficulty breathing, and coughing up blood

Tests based on which type of infection seems likely

Other disorders and causes

Chronic kidney disease

Shortness of breath, difficulty breathing when lying down that is relieved when upright (orthopnea), and/or swelling

Blood tests of kidney function

Connective tissue disorder (for example, rheumatoid arthritis or systemic lupus erythematosus [lupus])

Fever, night sweats, weight loss, joint pain, rash, and/or other organ involvement (for example, effects on the heart or lungs)

Blood tests for abnormal antibodies

Deconditioning

A history of lack of exercise or being bedbound or hospitalized

Only a doctor's examination

Diabetes (sometimes, other symptoms often more prominent)

Excessive thirst, excessive urination, increased appetite, and unexplained weight gain or loss

Measurement of sugar (glucose) level in the blood after the person fasts overnight and sometimes glucose tolerance testing

Drugs: Antidepressants, older antihistamines, antihypertensives, diuretics that cause low potassium levels in the blood, muscle relaxants, recreational drugs, and sedatives

History of taking a drug known to cause fatigue

Only a doctor's examination

Giant cell (temporal) arteritis

Headache, pain in the jaw when chewing, pain when combing hair, loss of vision, and/or muscle pains in a person over 50

ESR and temporal artery biopsy

Mental health disorders: Anxiety, depression, drug addiction, panic disorder, or somatization disorder (physical symptoms caused mainly by anxiety)

Anxiety, sadness, loss of appetite, and unexplained sleep disturbance

With somatization disorder, an excessive preoccupation with physical symptoms

Only a doctor's examination

Multiple sclerosis

Fatigue worse with exposure to heat

Past symptoms of nervous system malfunction (such as numbness, loss of coordination, and weakness), particularly if people had more than one episode of symptoms

Brain and/or spinal cord MRI

Pregnancy

Cessation of menstrual periods, breast tenderness, morning sickness, and abdominal swelling

Pregnancy test

Sleep disorders

Excessive daytime sleepiness, frequent awakenings, breathing interruptions during sleep, difficulty falling asleep, unrefreshing sleep

Sleep testing (polysomnography or home testing)

Underactive thyroid gland (hypothyroidism)

Inability to tolerate cold, weight gain, constipation, and coarse skin

Blood tests of thyroid function

Undernutrition

Weight loss

Sometimes loss of appetite, foul-smelling stool, abdominal pain, or a combination

A doctor's examination

Sometimes other tests

Disorders of unknown cause

Chronic fatigue syndrome (now sometimes called myalgic encephalomyelitis or systemic exertion intolerance)

Sore throat, sleep that is not refreshing, difficulty with concentration or short-term memory, muscle aches, joint pain, headaches, and/or tender lymph nodes in the neck or under the arms

Only a doctor's examination

Fibromyalgia

Long-standing and widespread muscle and bone pain in areas outside the joints, trigger points, lower abdominal pain, gas, bloating, constipation or diarrhea, migraines, and anxiety

Only a doctor's examination