One of the most common complaints 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 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 most common causes of fatigue and which blood tests can help you zero in on the cause.

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, exercising less, eating healthier, and reducing stress. But what happens when you’re still tired all the time even after a full night’s sleep, cutting the sugar out of your diet, and consistently meditating? 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 of the most common causes are deficiencies in iron, vitamin B12, and folate (vitamin B9).

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)
  • Lowering homocysteine, which reduces heart disease risk
  • Normal brain function
  • Immune system development and immune responses

Low iron levels can be caused by diets low in iron, gut issues that cause poor absorption of iron (e.g. Celiac disease), and chronic blood loss [7].

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

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 your 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. Because vitamin B12 is only found in animal foods, people eating vegan and vegetarian diets are at an increased risk of becoming deficient. Pregnant women are also at an increased risk due to the needs of the developing baby. As we age, we make substantially less stomach acid. Because stomach acid is needed to absorb vitamin B12, deficiency is more common in elderly people. Other causes of deficiency include gut issues and autoimmune diseases such as pernicious anemia that prevent vitamin absorption [15, 16, 17, 16].

Blood Test: You can test your total vitamin B12 levels. 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
  • Anti-oxidant purposes

The main cause of folate deficiency is 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. You can also check the levels of free T3 and free T4.

If you have a higher TSH level and are experiencing symptoms of hypothyroidism, you may want to 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. Only a few foods contain vitamin D naturally and fortified foods are unlikely to satisfy your daily requirements [27, 28].

Vitamin D has traditionally been recognized for its role in calcium metabolism and bone health. However, deficiency is linked to an increased risk of high blood pressure, Alzheimer’s disease, autoimmune disease, and many types of cancer. 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.

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].

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, then you’ll want to test your free testosterone 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]. Blood zinc, magnesium, and 25-hydroxyvitamin D are three other tests that you can easily get – low levels cause hormonal imbalances.

Do You Suffer From Chronic Fatigue?

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About the Author

Will Hunter

BA (Psychology)
Will received his BA in Psychology from the University of California, Los Angeles. 
Will's main passion is learning how to optimize physical and mental performance through diet, supplement, and lifestyle interventions. He focuses on systems thinking to leverage technology and information and help you get the most out of your body and brain.

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