You slept through the night, but by midmorning you’ve read the same email three times. The walk you usually take at lunch feels like more effort than it should. Maybe you’re struggling to stay awake or concentrate. If this sounds familiar, you’re not alone. In fact, you’re among the majority of people who often feel fatigued. But how can you find out what’s behind that “depleted” feeling?1
Your blood holds a wealth of information about how your body functions. Different blood tests measure different aspects of your health, from nutrient levels and hormone activity to the oxygen-carrying capacity of your red blood cells. Alongside your symptoms and health history, these results can help you and your provider investigate what’s causing your fatigue.12
Below are three common blood tests that may help clarify why you feel drained. Accessing these tests may be easier than you think. Depending on your location, you may be able to purchase these tests directly without obtaining an order from your doctor. For this article, we’ll discuss testing as part of a collaborative review of your symptoms and health history with your primary care provider (PCP).11
Ferritin and CBC Tests
We start with anemia, one of the most common causes of fatigue. A global study estimates that it affects nearly one-quarter of people worldwide as of 2021. It is especially prevalent among women, with the World Health Organization estimating that 30.7% of women ages 15–49 have anemia.8,9
To check for anemia, a provider will often order a complete blood count (CBC), which measures hemoglobin, the protein in your red blood cells that carries oxygen. A low hemoglobin level is a key finding used to diagnose anemia. Even when hemoglobin is within the reference range, a ferritin test may also be useful if iron deficiency is suspected. Ferritin reflects how much iron your body has stored and typically falls before hemoglobin does. That’s why a provider may check ferritin even when your CBC looks normal, especially if you have a history of heavy periods or other blood loss.2,3,4
If your ferritin is low, the next question is why. Heavy periods, blood loss from the digestive tract, and difficulty absorbing iron are possible causes. Inflammation can also raise ferritin and make the result harder to interpret. Depending on your history, your PCP may check other iron measures to further investigate why your iron stores are low.3,4
Also Read: Understanding Pain and Inflammation: Your Body’s Alert System
Vitamin B12 Test
Another top culprit of fatigue is nutrient deficiency, in particular, vitamin B12. The NIH reports that low or borderline-low B12 levels have been found in up to 40% of some Western populations, particularly among people with low intakes of B12-rich foods. If you’re feeling sluggish, your provider may consider a vitamin B12 blood test. Because B12 supports nerve function, deficiency can also cause numbness or tingling in the hands or feet. These symptoms can develop even when a blood count shows no anemia. Tell your provider if you eat a vegan diet, take the diabetes medication metformin, or have had stomach or intestinal surgery, since each can increase your risk of low B12.5
If your B12 level is borderline or your provider needs to clarify a possible deficiency, they may also order a methylmalonic acid (MMA) test. This test measures a substance that can build up when the body has too little B12. Kidney function can also affect MMA levels, so your provider will interpret the results alongside your B12 level and health history.5
Thyroid Function Tests
If you’ve been feeling unusually tired and reaching for a sweater when everyone else is comfortable, mention these changes to your provider. Fatigue and greater sensitivity to cold can occur with an underactive thyroid, called hypothyroidism (an overactive thyroid is called hyperthyroidism). A provider often starts with a thyroid-stimulating hormone (TSH) test. If the result is abnormal, a free T4 test can help show whether your thyroid is making enough thyroid hormone.6,7
Some tests include both TSH and free T4, allowing multiple markers to be measured together. When two or more tests are combined, they are considered a blood panel rather than a blood test. Although TSH is often the starting point, many providers will include free T4 and additional thyroid markers such as free T3. Your provider can help you decide which thyroid testing is appropriate if thyroid issues are suspected.7,10
TSH and free T4 measure different things. TSH is the signal the pituitary gland, a pea-sized gland at the base of your brain, sends to the thyroid. Free T4 measures the amount of thyroid hormone available to your tissues. In the most common form of an underactive thyroid, the pituitary increases its signal as thyroid hormone levels fall. High TSH paired with low free T4 typically indicates hypothyroidism.6,7
Be a Proactive Participant in Your Healthcare
If fatigue or low energy keeps interfering with your day, make an appointment with your provider and arrive prepared with a few concrete examples. When did your symptoms start? Are you waking up tired or running out of energy after activities you used to handle easily? Mention any changes in sleep, medications, supplements, or menstrual bleeding. These details help your provider decide which tests are worth ordering and what else to investigate.1,4
If these feelings of depletion resonate with you, ask your PCP whether any of these tests are appropriate. If bloodwork doesn’t explain your fatigue, your provider may review your diet, consider a sleep study for possible sleep apnea, or assess whether a medication could be contributing. Staying actively involved and working alongside your provider can help you understand the next steps and may just help you feel more like your old self.1
Also Read: How to Manage Chronic Fatigue and Get Your Life Back
Sources
- American Family Physician on evaluating fatigue in adults
- MedlinePlus on the complete blood count
- NIH Office of Dietary Supplements on iron
- MedlinePlus on iron deficiency anemia
- NIH Office of Dietary Supplements on vitamin B12
- NIDDK on underactive thyroid
- NIDDK on thyroid blood tests
- Global Burden of Disease Study on anemia prevalence in 2021
- WHO global anemia estimates, 2025 edition
- American Thyroid Association on reflex free T4 testing
- Quest on purchasing tests and included physician oversight
- MedlinePlus on what blood tests can reveal
Blair Ramsing is the co-owner of Baseline Bloodwork, a direct-access blood draw facility in Fort Collins, Colorado.
Disclaimer: This article is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before making decisions about your health, including changes to your diet, exercise, supplements, or medications.
