Exhausted all the time? Which tests make sense first
Which tests are worth doing first?
UK NICE guidance for suspected ME/CFS lists these initial tests to look for other causes: urinalysis, full blood count, kidney function and electrolytes, liver function, thyroid function, ESR or plasma viscosity, CRP, calcium and phosphate, HbA1c, ferritin, celiac screening and creatine kinase. [Source 1]
Full blood count (CBC)
Guideline-backedWhat it can tell you
Anemia, infection, and some blood disorders.
What it can't tell you
Low iron before it causes anemia — that's what ferritin is for.
Source: Source 1, 2
Ferritin
Guideline-backedWhat it can tell you
Low iron stores, even before anemia.
What it can't tell you
Much on its own if you have inflammation, which can push ferritin up and hide a deficiency. Worth knowing: cutoffs vary — the WHO uses below 15 µg/L in otherwise healthy adults, while the American Gastroenterological Association uses below 45 ng/mL in people with anemia.
Source: Source 1, 3, 4
TSH (± free T4)
Guideline-backedWhat it can tell you
An under- or overactive thyroid. The American Thyroid Association calls TSH the best first test.
What it can't tell you
The cause — antibodies answer that, when relevant.
Source: Source 5
Kidney, liver, calcium, HbA1c
Guideline-backedWhat it can tell you
Diabetes, kidney or liver problems, and calcium imbalances.
Source: Source 1
Celiac blood test
Guideline-backedWhat it can tell you
Celiac disease, which can cause fatigue even without gut symptoms. You need to be eating gluten for it to be accurate.
Source: Source 1
CRP and ESR
Guideline-backedWhat it can tell you
General inflammation or infection.
What it can't tell you
Which condition is causing it.
Source: Source 1
Vitamin B12
Useful in specific situationsWhat it can tell you
B12 deficiency — more relevant with anemia, a vegan diet, or long-term use of metformin or acid-reducing medicines.
Source: Source 2
Vitamin D
Not validated / not recommended(as a routine test)
What it can tell you
Deficiency in specific conditions, such as osteoporosis or malabsorption.
What it can't tell you
The cause of fatigue. The Endocrine Society's 2024 guideline suggests against routine vitamin D testing in healthy adults.
Source: Source 6
Sleep study
Useful in specific situationsWhat it can tell you
Sleep apnea or other sleep disorders, when symptoms or a screening questionnaire point to it.
Source: Source 2
Pregnancy test
Useful in specific situationsWhat it can tell you
Pregnancy, for anyone who could be pregnant.
Source: Source 2
Why are my tests normal when I'm this tired?
Because fatigue has many causes that blood tests can't see — sleep disorders, POTS, ME/CFS, depression, medication effects, and the cumulative load of chronic pain. A US family-medicine review found that lab tests ordered without a specific reason change management in only about 5% of people. That doesn't make the fatigue less real. It means the next step is usually a closer look at your history, not more blood tests. [Source 2]
What about "adrenal fatigue" and functional panels?
See our functional tests page. In short: adrenal fatigue isn't a recognized diagnosis, and most functional panels marketed for fatigue haven't been validated.
What sources does this page use?
- NICE. Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management (NG206). 2021. https://www.nice.org.uk/guidance/ng206
- Latimer KM, Gunther A, Kopec M. Fatigue in adults: evaluation and management. Am Fam Physician. 2023;108(1):58–69.
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020.
- Ko CW, et al. AGA clinical practice guidelines on the gastrointestinal evaluation of iron deficiency anemia. Gastroenterology. 2020;159(3):1085–1094.
- American Thyroid Association. Thyroid function tests. https://www.thyroid.org/thyroid-function-tests/
- Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.
How should I cite this page?
Niehaus L. Exhausted all the time? Which tests make sense first. The Lily Perspective. Published October 2026. Updated October 2026. https://lilyperspective.com/toolkit/lab-tests/fatigue
© 2026 Lily Niehaus. All rights reserved. You're welcome to link to this page or quote short passages with credit to The Lily Perspective.