What causes extreme fatigue?

Extreme fatigue can affect both men and women for a large range of reasons. The MediFind Medical Team has summarized 10 potential causes for extreme fatigue below in order from most to least likely based on our data. Learn more about how MediFind works here. You can also enter your symptoms into MediFind’s Symptom Checker to receive more customized results.

Chronic Fatigue Syndrome

When fatigue does not improve with rest and worsens with activity, it could be a syndrome called myalgic encephalomyelitis, also known as chronic fatigue syndrome. This condition can last for at least six months and has no known cause. A neurologist will perform multiple examinations to rule out other possible conditions. Find a neurologist near you here.


Sometimes called mono, mononucleosis refers to a viral infection characterized by fatigue, headaches, and a sore throat that progressively worsens. Some patients experience a measles-like rash after undergoing antibiotic treatment. An infectious disease doctor would mostly focus on treating the symptoms with steroids like prednisone, as antiviral drugs are not particularly effective. Oftentimes the best course of action is to rest and wait it out. Find an infectious disease doctor near you here.


Extreme fatigue is one of many outcomes of dehydration, which is when the body lacks water and fluids. Cases of severe dehydration, involving extreme fatigue, are medical emergencies and should be taken seriously. A patient can become dehydrated due to simply not drinking enough fluids but excessive sweating, vomiting, diarrhea, or urination can also contribute. A primary care doctor will ensure rehydration occurs through an IV drip, which pumps fluid into the veins. Find a primary care doctor near you here.

Crohn’s Disease

As a consequence of severe diarrhea, brought on by Crohn’s disease, a patient can experience severe fatigue due to dehydration. Crohn’s disease occurs from chronic inflammation of the digestive tract and can progressively worsen. The inflammation is not limited to the digestive tract, developing on the skin, eyes, joints, liver, and bile ducts. In children, it may even lead to delayed sexual development and growth. A gastroenterologist will focus on reducing the symptoms of the disease, as there is no known cure. Progression can also be limited via immune system suppressors. Find a gastroenterologist near you here.

Vitamin B12 Deficiency Anemia

When the body lacks healthy red blood cells, extreme fatigue is among many symptoms they will experience due to a lack of oxygen arriving to organ systems. This anemia comes in many forms. Vitamin B12 anemia is a lack of vitamin B12 that results in a lower red blood cell count. This can happen due to dietary factors, such as vegetarianism or pregnancy, but can also relate to a previous infection or surgery. A hematologist would prescribe vitamin B12 supplements. Find a hematologist near you here.

Myasthenia Gravis

When the immune system targets the muscles and nerves, it is called a neuromuscular disorder or myasthenia gravis and results in muscle weakness alongside extreme fatigue. Rest improves the muscle weakness, which distinguishes it from chronic fatigue syndrome. A neurologist will prescribe neostigmine or pyridostigmine to improve neuromuscular communication and steroids to inhibit the autoimmune response. A thymectomy to remove the thymus, and thus eliminate the autoimmune response, is a drastic but working solution. Find a neurologist near you here.

Pulmonary Hypertension

High blood pressure in the lungs is referred to as hypertension and can result in extreme fatigue. The right side of the heart has to pump harder than normal, which causes it to inflame and eventually fail; a condition called corpulmonale. Besides fatigue, patients may also experience chest pain, bluish color of the lips or skin, swelling of the limbs, and general weakness. A hematologist will focus on lessening lung damage through the use of blood thinners and oxygen therapy. If the lung is too far-gone, a lung transplant may be necessary. Find a hematologist near you here.

Childhood Iron Deficiency Anemia

Much like vitamin B12 anemia, iron deficiency anemia refers to a lack of iron in the body. This can cause extreme fatigue, as iron is required to make healthy red blood cells, which then provide oxygen to muscles and organs. Childhood anemia involves a lifelong history with the illness that starts at a young age. Other symptoms include shortness of breath, headaches, and lack of appetite. A hematologist will try to overcome the lack of iron by dietary means but can also provide iron supplements. It is important that only the doctor prescribe the iron supplements, as iron can be toxic to children. Find a hematologist near you here.

Lyme Disease

Transmitted via blacklegged ticks, Lyme disease is caused by a bacterium called Borrelia burgdorferi, which uses the ticks as a vector for transmission. It is characterized as extreme fatigue but specific to particular locations and whether the patient has traveled outdoors or hiked. Some patients may be more susceptible to Lyme disease than others may. An infectious disease doctor will prescribe oral antibiotics for early stages but there is no complete cure. Find an infectious disease doctor near you here.


Influenza is a viral infection that targets the nose, throat, and lungs and has a long history of causing widespread pandemics. Doctors label it by its structural organization, which has changed due to evolution. More common forms of influenza are swine flu (H1N1) or influenza A or B. Like most infectious diseases, patients will experience extreme fatigue alongside very high fevers. They will have body aches and a difficulty in getting out of bed. A primary care doctor will emphasize lowering the fever to safer levels by using over the counter medicine, though they avoid using aspirin. They recommend resting and regular hydration. Find a primary care doctor near you here.

Understanding the Results

These results are based on the most likely conditions for a 20 to 40 year-old patient that is living in the United States. Our data shows that 96% of the time, this symptom is related to one of the 10 most likely causes. Location and age can also contribute to different results. Use our Symptom Checker to add your information and get your custom results.

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Differences in Age Groups

Extreme fatigue appears as a symptom generally for infectious diseases but can also occur in sources of anemia. These conditions are not specific to any age group, though children have a tendency to infected more often.

Differences among Men and Women

There is no significant difference in prevalence among men and women. However, women have a higher incidence of anemia, particularly when pregnant.

Last Updated: December 08, 2022

Published By: MediFind Medical Staff