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5 Common Causes of Chronic Fatigue + Helpful Lab Tests

Written by Biljana Novkovic, PhD | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Biljana Novkovic, PhD | Last updated:

A common complaint doctors hear from their patients is fatigue. The type that doesn’t seem to go away no matter what you do. Fatigue can affect every aspect of our lives and prevent us from living life to the fullest. And because it can be caused by many different factors, finding the source of your fatigue can be a frustrating, costly, and time-consuming endeavor. Read on to discover the 5 common causes of fatigue and which blood tests can help you zero in on them.

What is Chronic Fatigue?

Fatigue is our body’s way of letting us know that we need to rest and recover. It happens after intense exercise or sleep loss. Normally, we can beat fatigue by sleeping more, letting our bodies recover after exercise, eating healthier, and reducing stress. But what happens when you’re still tired all the time even after a full night’s sleep and cutting the sugar out of your diet? This is when fatigue becomes chronic. With chronic fatigue, you don’t just feel physically tired and weak. You also lack the motivation to accomplish daily tasks, can’t concentrate, and may even feel slightly depressed.

If you can relate to this, then it’s time to dig deeper to see what’s really going on. By taking specific lab tests, you can uncover the source of your fatigue and start taking steps to get your mood, focus, and energy back.

Common Causes of Chronic Fatigue

1) Anemia

Anemia is a condition in which your body isn’t producing enough red blood cells or enough hemoglobin (the protein that carries oxygen). The main symptom of anemia is fatigue. Anemia can be caused by many different things, but three common nutritional causes are deficiencies in iron, vitamin B12, and folate (vitamin B9).

Your doctor can diagnose anemia, taking into account your medical history, symptoms, and test results such as red blood cells (RBC), hemoglobin, and hematocrit.

Iron Deficiency

One of the most common causes of fatigue is iron deficiency. Indeed, it’s the most common nutrient deficiency worldwide. Iron is a mineral that plays an essential role in the body. Most of the iron in the body is found in hemoglobin, where it helps to carry oxygen to cells, which use it to produce energy. Iron is also needed for [1, 2, 3, 4, 5, 6]:

  • Transporting and storing oxygen in muscles (as part of the protein myoglobin)
  • Normal brain function
  • Immune system development and immune responses

Low iron levels can be caused by [7]:

  • Diets low in iron
  • Gut issues that impair absorption of iron (e.g. Celiac disease)
  • Chronic blood loss (e.g. heavy menstrual bleeding)

Blood Test: Because blood iron fluctuates daily and increases after you eat iron-rich foods, ask your doctor about a ferritin test instead. Ferritin is a protein that stores and transports iron and is a much more accurate indicator of how much iron is in the body. Low ferritin levels strongly suggest iron deficiency as there are only a few other conditions that decrease ferritin [8, 9, 10]. Talk with your doctor to find out more.

Vitamin B12 Deficiency

Like iron, vitamin B12 is also needed to make red blood cells, making it a crucial nutrient for energy production. It also plays a key role in making DNA and is required for the brain and nervous system to work properly [11, 12, 13, 14].

Vitamin B12 deficiency often takes years to develop as the body is able to store large amounts in the liver. People at a higher risk of vitamin B12 deficiency include [15, 16, 17]:

  • People eating vegan and vegetarian diets, because vitamin B12 is only found in animal-derived foods
  • Pregnant women, due to the needs of the developing baby
  • Seniors, because nutrient absorption decreases with age
  • People with gut issues that prevent vitamin absorption

Blood Test: You can ask your doctor if you should test your vitamin B12 levels. Total vitamin B12 levels are most commonly tested. There are proponents of testing the active vitamin B12, but research is conflicted on whether it improves the detection of vitamin B12 deficiency over total vitamin B12 while being more expensive and hard to perform [18].

Folate (Vitamin B9) Deficiency

Folate is another key vitamin needed to make your red blood cells. It also is needed for [19, 20, 21, 22]:

  • Amino acid metabolism
  • Making DNA
  • Maintaining brain and nervous system health
  • Lowering homocysteine, which reduces heart disease risk

Folate deficiency is rare. It can happen in people not eating enough fruits and vegetables. Alcoholics and lactating mothers also have an increased risk of deficiency [23].

Blood Tests: You can test either your serum folate or red blood cell folate (RBC folate). Folate is more concentrated in red blood cells, making RBC folate a better indicator of how much total folate is stored in your tissues [24].

2) Poor Thyroid Function (Hypothyroidism)

The thyroid is a gland that controls how fast your body’s metabolism runs. Hypothyroidism is a condition in which your thyroid isn’t making enough thyroid hormones. If you have hypothyroidism, you will experience fatigue, inability to lose weight, hair loss, and sensitivity to cold.

Nearly 90% of hypothyroid cases are caused by an autoimmune disease called Hashimoto’s thyroiditis, in which the body attacks the thyroid gland and causes inflammation. Other causes include iodine and selenium deficiencies, certain medications, and radiation therapy for cancer [25].

Blood Tests: If you have symptoms of hypothyroidism, doctors will usually test your thyroid-stimulating hormone (TSH) first. TSH is a hormone that tells the thyroid gland to make thyroid hormones. They may also check the levels of free T3 and free T4.

If needed your doctor will check your thyroid antibodies: thyroid peroxidase antibodies (TPOAbs) and thyroglobulin antibodies (TgAbs). TPOAbs are found in more than 90% and TgAbs are found in 80% of Hashimoto’s patients [26].

3) Vitamin D Deficiency

One of the most common nutrient deficiencies besides iron deficiency is vitamin D deficiency, which affects nearly a billion people worldwide. The association of sun exposure to skin cancer and skin damage has caused people to dramatically reduce their time in the sun. But as sunlight provides 50-90% of your vitamin D requirements, you need a certain amount of sensible sun exposure [27, 28].

Only a few foods contain vitamin D naturally and fortified foods are unlikely to satisfy daily requirements [27, 28].

Vitamin D has traditionally been recognized for its role in calcium metabolism and bone health. Many studies have also found that deficiency is associated with fatigue. Unsurprisingly, correcting vitamin D deficiency improves fatigue in healthy people and people with various diseases [29, 30, 31, 32, 33, 34].

Blood Test: The 25-Hydroxyvitamin D test is the best measure of your vitamin D stores.

4) Poor Liver Function

The liver is responsible for helping with digestion, detoxifying drugs, and chemicals, making proteins, and storing nutrients like iron and vitamins A and B12. The liver plays a crucial role in maintaining your energy levels and fatigue is one of the most common complaints in people with chronic liver disease [35, 36].

Non-alcoholic fatty liver disease (NAFLD), the buildup of fat in the liver, is by far the most common cause of poor liver function and liver disease. It’s caused by diabetes, obesity, and high cholesterol and triglyceride levels. Drinking too much alcohol, viral infections, autoimmune disease, and toxins can all take a toll on your liver and lead to liver disease [37, 38, 39]

Blood Tests: The following tests can show you how well your liver is functioning: aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), and bilirubin. These are all tested as part of a comprehensive metabolic panel.

If these are elevated, then this is a sign your liver may not be functioning as well as it should be. Your doctor will interpret these tests in conjunction with your signs, symptoms, medical history, and other test results.

5) Low Testosterone Levels

Testosterone is a hormone that is vital to maintaining high energy levels not just in men, but women as well. One of the most common complaints in people with low testosterone is a lack of energy and motivation.

Unfortunately, testosterone levels decline by 1-2% a year after peaking in the ’20s in both men and women [40, 41, 42].

If you’re young and experiencing low testosterone levels, then you may be suffering from deficiencies in zinc, magnesium, or vitamin D [43, 44, 45]. Work with your doctor or another health care professional to get an accurate diagnosis.

Blood Tests: Doctors usually test your total testosterone first if you’re showing symptoms of low testosterone, such as fatigue, lack of libido, and depression. However, just testing total testosterone may miss deficiencies. That’s because two-thirds of testosterone is bound to sex hormone-binding globulin (SHBG) and is unable to have any effects on the body.

If your total testosterone is normal, and you’re still experiencing symptoms of deficiency, ask your doctor for a free testosterone test as well. This is the testosterone that isn’t bound to anything and is able to improve your energy and strength, increase fat loss, and improve your mood [46, 47].

About the Author

Biljana Novkovic

Biljana Novkovic

Biljana received her PhD from Hokkaido University.
Before joining SelfHacked, she was a research scientist with extensive field and laboratory experience. She spent 4 years reviewing the scientific literature on supplements, lab tests and other areas of health sciences. She is passionate about releasing the most accurate science and health information available on topics, and she's meticulous when writing and reviewing articles to make sure the science is sound. She believes that SelfHacked has the best science that is also layperson-friendly on the web.


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