5 min - BioSense Team - 2026-06-24
- level:understanding
- fatigue
- ferritine
- thyroïde
- biomarqueurs
Fatigue that settles in over several weeks deserves more than a shrug. A well-targeted blood test, read in your personal context, opens up concrete avenues to explore with your doctor. This article reviews the priority 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.
When should you talk about persistent fatigue?
We talk about persistent fatigue — doctors often use the term asthenia — 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 more than four 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.
After 40, several biological causes become statistically more common: iron deficiencies, early thyroid disorders, prediabetes, vitamin D or B12 deficits. A well-targeted blood test makes it possible to lay out factual elements — not to self-diagnose, but to prepare a more useful conversation with your doctor.
Key point — Fatigue that lasts more than four weeks, or that affects daily life, deserves to be documented. Age is not a sufficient explanation in itself.
Five major groups of biomarkers to explore first
1. Iron status: ferritin, haemoglobin, 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 anaemia appears. Haemoglobin, measured in the complete blood count (CBC), indicates whether anaemia is already present. Transferrin saturation completes the picture by assessing iron availability.
Good to know: a ferritin level in the low-normal range (for example around 20 to 30 µg/L) doesn't always trigger an automatic alert on the laboratory report, yet it can be enough to explain fatigue in some people. To go further, see Ferritin: what this marker reveals about your iron stores.
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. In case of an abnormality, your doctor can add free T4 and anti-TPO antibodies.
3. Blood glucose and HbA1c
Prediabetes or early diabetes can show up as diffuse fatigue, often after meals. Fasting blood glucose gives a snapshot at a single point in time; glycated haemoglobin (HbA1c) reflects the average over the last two to three months. Guidelines place prediabetes between 5.7% and 6.4% HbA1c, and diabetes from 6.5%.
Key point — Iron, thyroid and blood glucose are the three families to explore first in the majority of cases of persistent fatigue. None of these investigations is carried out without prior medical advice.
4. Inflammation: CRP, erythrocyte sedimentation rate
Low-grade chronic inflammation — even without an identified cause — can sustain a state of fatigue. CRP (C-reactive protein) and the erythrocyte sedimentation 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 deficiency is very common in France, particularly in winter and in people with little sun exposure. It can contribute to a feeling of fatigue, diffuse muscle pain and being generally below par. Vitamin B12 is essential for the production of red blood cells and for proper neurological function; a deficiency — more common after 60, in strict vegetarians or in people on certain stomach medications — 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 averages 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 laboratory can, in reality, be suboptimal for you.
A concrete example: a ferritin level of 25 µg/L will be flagged as normal on most reports. Yet for an active 48-year-old woman, in premenopause, complaining of fatigue for three months and still having heavy periods, this figure tells another story. 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 laboratory range is a statistical average, not a personalised 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 (HAS, ANSM, Santé Publique France, INSERM, WHO, international learned societies). 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.
Preparing for your medical appointment: three useful questions
If persistent fatigue concerns you, here are three questions to prepare for your next consultation:
- Given my results and my context (age, history, lifestyle, treatments), are further biological investigations indicated?
- My values are within the laboratory range, but are they optimal for me?
- When should I repeat a test to track how things evolve?
This preparation saves time during the consultation and steers the conversation toward what really matters for you.
*This article is for informational purposes only and is neither medical advice nor a diagnosis. For any questions about your health, consult your doctor.*
Import your blood test into BioSense and receive a plain-language explanation of each of your biomarkers, contextualised to your profile, with tracking over time. Your data is hosted in France on infrastructure certified for hosting health data (HDS).
[→ Import my report](/en/onboarding/upload)
Sources and methodology
- French Health Insurance (Ameli) — Asthenia: the medical consultation and treatment.
- High Authority for Health (HAS) — Fatigue and exhaustion in prolonged symptoms of Covid-19 (2023).
- HAS — Choosing iron metabolism tests when iron deficiency is suspected (Appropriate use fact-sheet, 2011).
- HAS — Care pathway guide: Type 2 diabetes in adults.
- American Diabetes Association (ADA) — Standards of Care in Diabetes (annual update), HbA1c thresholds.
- HAS and the French Society of Endocrinology (SFE) — Subclinical hypothyroidism in adults: diagnosis and management (2007).
- HAS — Management of hypothyroidism in adults (good practice recommendation, 2022).
- GRIO (Research and Information Group on Osteoporosis) — Recommendations on vitamin D.
- Anses — Nutritional reference values for vitamins and minerals.
- Santé Publique France — Esteban study 2014-2016, nutrition component (biological measurements: vitamins and minerals, December 2019).
*BioSense methodology: our content draws on official French guidelines (HAS, ANSM, Santé Publique France, INSERM, Assurance Maladie, Anses), European guidelines (ESC, EASD, NICE) and international ones (WHO, ADA, AHA, NIH), as well as specialised learned societies (SFE, SFBC, GRIO). It is reviewed with an educational approach before publication.*