Fatigue causes could be related to an illness; have your physician rule out the examples listed here.

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If you’re feeling worn out, run-down, and hopelessly fatigued, your first impulse might just be to “sleep it off.” But if it feels like chronic fatigue, causes can vary wildly—and could be warning signs of a serious medical issue.

So what causes lack of energy in us? All kinds of ailments and illnesses can be extreme fatigue causes. Your healthcare provider can guide you in determining whether your fatigue may be related to one of the following conditions.

Anemia: Most Common of Fatigue Causes

Fatigue is “the most common symptom of all types of anemia,” according to the National Institutes of Health. “Fatigue occurs because your body doesn’t have enough red blood cells to carry oxygen to its many parts.”

If you suffer from anemia, the red blood cells produced by your body have less hemoglobin than normal. Hemoglobin—the iron-rich protein in red blood cells—helps red blood cells carry oxygen from your lungs to the rest of your body. A shortage of hemoglobin and the resulting anemia brings on excess tiredness and also can cause shortness of breath, chest pain, and serious heart problems.

Celiac Disease

Celiac disease is known to cause not only abdominal pain, a foggy mind, joint pain, tingling of the extremities, and depression (among other issues), but fatigue. A related condition, non-celiac gluten sensitivity, is also known to be what causes fatigue in some. A celiac test will resolve the issue for you. If it turns out you’re intolerant of gluten, you’ll adopt a gluten-free diet

Cancer

Various types of cancer are known to be fatigue causes, and it’s not unusual for lethargy and tiredness to precede other symptoms. Increased levels of cytokines (chemicals your body produces when fighting an infection or cancerous cells) contribute to fatigue, as does the loss of appetite experienced by many people who develop cancer.

A lack of energy and loss of appetite doesn’t mean you have a type of cancer, but in combination with such other symptoms as mysterious pains, fevers, and lumps or swelling, they may be reason enough to consult your healthcare provider.

Cardiovascular Issues

It could be that cardiovascular problems are behind your fatigue. Studies of the relationship between fatigue and cardiac function indicate evidence of a link. One study of 515 female heart attack patients, average age 66, found that 70 percent of them reported heavy weariness that debilitated them, in most cases more than a month before their heart attack. Most common symptom reported by these patients: fatigue.

In fact, the medical world has long held that women and elderly heart attack patients are more likely to experience unusual fatigue than chest discomfort.

Fibromyalgia

Similar to chronic fatigue syndrome, fibromyalgia affects between 2 and 4 percent of Americans, studies show. Women tend to be more at risk than men for both. Each disorder can cause not only severe fatigue but muscle pain. Fibromyalgia can cause pain in spots referred to as “tender points”; the most bothersome, patients report, seem to affect the bottom of the neck and the area between the shoulder blades.

Hormonal Disorders

Research has shown that hormonal disorders can indirectly produce fatigue. Extreme low or high levels of certain hormones cause such conditions as hypothyroidism, diabetes, and Addison’s disease, all of which can create lethargy.

Nearly 10 million Americans have hypothyroidism, which becomes more of a risk as we age. And the American Association of Clinical Endocrinologists tells us that women are five times more likely than men to develop hypothyroidism. Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormone, according to the American Thyroid Association (ATA), and among its symptoms is extreme fatigue.

Multiple Sclerosis (MS)

An autoimmune disease that affects some 400,000 Americans and more than 2 million people internationally, multiple sclerosis also can be a fatigue cause. In fact, experts say that MS’s most debilitating symptom is unremitting fatigue, including brain fatigue that finds those effected unable to concentrate or engage in activities.

MS-related fatigue can be intense, can occur daily for half (or more) of your waking hours; can be exacerbated by humidity and heat; and tends to overcome a person without warning.

It could well be that your fatigue is unrelated to these and other serious medical issues. Feelings of lethargy may simply be a wake-up call to consistently get more rest, get more exercise, and eat a healthier diet.

Originally published May 2016 and updated.