Overview
The International Classification of Diseases (ICD-11) defines MG22 Fatigue as:
“A feeling of exhaustion, lethargy, or decreased energy, usually experienced as a weakening or depletion of one’s physical or mental resource and characterised by a decreased capacity for work and reduced efficiency in responding to stimuli. Fatigue is normal following a period of exertion, mental or physical, but sometimes may occur in the absence of such exertion as a symptom of health conditions.”
In an everyday context, “fatigue” has a much more casual usage. One might say, “after Breath of the Wild and Elden Ring, I have open-world fatigue” to mean that someone is “tired” of something in the sense of being bored or burnt out, or one might say, “I feel fatigued after a party” to just mean they’re extremely tired. These aren’t wrong uses of the term per se, but clinicians are interested in fatigue that’s persistent and interferes with daily life such that it may be associated with underlying health conditions:
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Hypothyroidism: an underactive thyroid leads to slow metabolic function that’s slow to process energy
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Hyperthyroidism: an overactive thyroid leads to higher metabolic function that burns through energy reserves quickly
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Diabetes Mellitus: an inefficient rate of processing glucose means cells are continuously starved for energy
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Addison’s Disease: Low cortisol production leads to poor management of stress and metabolic processes
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Hypopituitarism: dysfunction in the pituitary gland can cause multiple hormone disruptions
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Anemia: severe iron deficiency can lead to a lack of red blood cells that transport oxygen throughout the body
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Vitamin B12 Deficiency: a lack of B12 can hamper nerve function and the production of red blood cells
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Dehydration: the disruption of balance to fluids and electrical conduction between the nerves can lead to exhaustion
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Obstructive Sleep Apnea (OSA): apnea presents as sudden pauses in breathing when sleeping that can disrupt the quality of rest and levels of oxygen in the blood