5 min - BioSense Team - 2026-06-24
- level:understanding
- fatigue
- ferritine
- thyroïde
- biomarqueurs
Always tired after 40? Fatigue that lasts for weeks or weighs on daily life deserves to be explored with your primary care doctor. Blood work guided by your symptoms often looks at iron, the thyroid and blood sugar, then at inflammation and vitamins D and B12 depending on context. This article reviews these biomarkers and explains why a result "within range" can still tell you something.
You wake up tired. The weeks go by, the fatigue persists, without lack of sleep, work stress or the season being quite enough to explain it. Before putting it down to age, a blood test can provide concrete elements to discuss with your doctor. Here's what's useful to explore first, and what these markers really tell you about your health.
Why am I always tired? When to see a doctor about fatigue
We talk about persistent fatigue when the feeling of exhaustion sets in over time and doesn't go away with rest. Temporary fatigue, after a period of intense work or poor sleep, is normal. Fatigue that lasts for weeks, that affects your daily activities or that comes with other signs (breathlessness on exertion, loss of drive, concentration difficulties, unusual sensitivity to cold), deserves investigation. In American Family Physician's 2023 review, fatigue lasting six months or longer is considered chronic, but there is no need to wait that long to bring it up.
Possible causes include iron deficiency, thyroid disorders, prediabetes or diabetes, and vitamin B12 deficiency, but also sleep, mood and medicines. The same review offers a useful caution: blood work without a specific reason rarely helps, changing treatment in about 5% of patients. The goal is not to self-diagnose, but to prepare a more useful conversation with your doctor.
Key point - Fatigue that lasts for weeks, or that affects daily life, deserves to be documented. Age is not a sufficient explanation in itself.
Blood tests for fatigue: five groups of biomarkers to discuss
1. Iron status: ferritin, hemoglobin, transferrin saturation
Iron deficiency is a classic cause of fatigue, particularly in women before menopause. Ferritin reflects your iron stores in the body: a low level can be associated with marked fatigue, even before frank anemia appears. Hemoglobin, measured in the complete blood count (CBC), indicates whether anemia is already present. Transferrin saturation completes the picture by assessing iron availability.
Good to know: a ferritin of 20 to 30 ng/mL (20 to 30 µg/L) doesn't always trigger an alert on a lab report, yet the American Society of Hematology's 2026 guideline treats 30 ng/mL (30 µg/L) or lower as diagnostic of iron deficiency in adults, and it suggests higher cutoffs in people with inflammation. The WHO has long used a lower cutoff in healthy adults, so reports differ. For the opposite case, see High ferritin: the causes.
2. The thyroid: TSH as first-line
The thyroid regulates many energy mechanisms. Early hypothyroidism, even mild, can show up as fatigue, sensitivity to cold, mental slowing or unexplained weight gain. TSH (thyroid-stimulating hormone) is the first-line marker: its elevation points to a slowed thyroid function. If it is abnormal, your doctor can add T4 and thyroid antibody tests.
3. Blood sugar and A1C
Prediabetes or early diabetes can show up as diffuse fatigue. Fasting blood glucose gives a snapshot at a single point in time; hemoglobin A1C (HbA1c) reflects the average over the last two to three months. In the US, prediabetes corresponds to an A1C of 5.7% to 6.4% (39 to 46 mmol/mol), and diabetes to an A1C of 6.5% (48 mmol/mol) or above, usually confirmed by a repeat test.
Key point - Iron, thyroid and blood sugar are common starting points when fatigue persists. Which tests make sense is decided with your doctor, based on your history and exam.
4. Inflammation: CRP, sed rate
Inflammatory conditions can go with fatigue. CRP (C-reactive protein) and the erythrocyte sedimentation rate (ESR, or sed rate) are the routine markers. A slightly elevated CRP with no infection context deserves a discussion with your doctor to look for its origin.
5. Vitamins: vitamin D and B12
Vitamin D levels fall in winter and in people with little sun exposure, but its link with fatigue is not well established, and routine testing is not recommended for everyone: the USPSTF (2021, I statement) finds the evidence insufficient for screening adults without symptoms, and the Endocrine Society (2024) found no trial evidence supporting routine testing in the general population. Your doctor can tell you whether it applies to your situation. Vitamin B12 is essential for the production of red blood cells and for proper neurological function. A deficiency, more likely with diets low in animal products, celiac or Crohn's disease, alcohol use disorder, past digestive surgery or certain medicines, can show up as fatigue, tingling or low mood.
Why a result "within range" can still explain your fatigue
The reference ranges printed next to your results are statistical intervals calculated on large populations. They are not tailored to your age, sex, history, current treatments or activity level. A result strictly "within range" for the lab can, in reality, be suboptimal for you.
A concrete example: a ferritin level of 25 ng/mL (25 µg/L) may still be printed as normal on a lab report. Yet for an active 48-year-old woman, in perimenopause, complaining of fatigue for three months and still having heavy periods, this figure tells another story, and it falls under the 2026 American Society of Hematology threshold. It is precisely this difference, between the raw reading of the numbers and putting them in context, that changes the conversation with her doctor. On this point, see Blood test reference ranges: why "within range" isn't enough.
Key point - A lab range is a statistical interval, not a personalized target. Putting your results in context (age, sex, history, lifestyle, treatments) changes how they read.
The BioSense method
BioSense helps you understand your results in the light of your personal context and official medical sources (US authorities and medical societies, the WHO). The service cross-references your health questionnaire, age, history, current treatments, lifestyle, reason for the prescription, with each biomarker, and enables tracking over time rather than a reading at a single moment.
*ChatGPT answers you. BioSense knows you.* *BioSense, your prevention companion.* BioSense does not provide a diagnosis and does not replace your doctor. It is a prevention companion that helps you understand, ask the right questions and act day to day.
Persistent fatigue: three questions to ask your doctor
If persistent fatigue concerns you, here are three questions to prepare for your next visit:
- Given my results and my context (age, history, lifestyle, treatments), are further biological investigations indicated?
- My values are within the lab range, but are they optimal for me?
- When should I repeat a test to track how things evolve?
This preparation saves time during the visit and steers the conversation toward what really matters for you.
Frequently asked questions
What blood tests should I ask for if I'm always tired?
The choice is your doctor's and depends on your symptoms, but it often covers a CBC, ferritin, TSH, blood sugar or A1C, and sometimes CRP or vitamin B12.
What vitamin deficiency causes fatigue?
Iron and vitamin B12 deficiencies are among the classic causes of fatigue; the role of vitamin D is less clear. A low ferritin level can come with marked fatigue even before anemia appears. Only a doctor can link a number to your symptoms.
How long should fatigue last before seeing a doctor?
There is no fixed official delay. Fatigue that lasts for several weeks is worth raising with your doctor. Do not wait if it interferes with your daily activities or comes with other signs, such as breathlessness on exertion.
What if I'm tired but my blood work is normal?
Tell your doctor, describing what has changed and since when. Sleep, mood, medicines and other conditions are also explored. A value within the lab range does not rule out every avenue: a low ferritin level inside the range, for example, does not always trigger an alert on the report.
*This article is for informational purposes only and is neither medical advice nor a diagnosis. For any questions about your health, consult your doctor.*
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Sources and methodology
International
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. 2020.
United States
- Latimer KM, Gunther A, Kopec M. Fatigue in adults: evaluation and management. American Family Physician 2023;108(1):58-69.
- American Society of Hematology. ASH sets new standards for diagnosing iron deficiency. 2026. Guideline: Blood Advances 2026, DOI 10.1182/bloodadvances.2025015950.
- NIDDK. The A1C test and diabetes.
- American Diabetes Association. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes 2026. Diabetes Care 2026;49(Supplement 1).
- MedlinePlus. Ferritin blood test; TSH test; Vitamin B test; C-reactive protein test.
- USPSTF. Vitamin D deficiency in adults: screening. I statement, 2021.
- Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism 2024;109(8):1907-1947.
- Kasahara AK, Singh RJ, Noymer A. Vitamin D (25OHD) serum seasonality in the United States. PLoS One 2013;8(6):e65785.
*BioSense methodology: our content is based on guidance from US authorities and medical societies (such as the USPSTF, CDC, NIH, FDA, ACC/AHA and ADA), UK bodies (NICE, NHS), European societies and the WHO. Sources are listed by country, and differences between countries are stated when they matter.*