In short
- The most common causes that blood can show: iron deficiency, thyroid, B12 or folate deficiency, blood sugar, silent inflammation, coeliac disease and kidney or liver damage.
- Ferritin without CRP is unreliable — with inflammation it is falsely high and hides a deficiency (WHO 2020).
- In women of childbearing age fatigue is most often explained by empty iron stores, even when the blood count is still normal.
- Blood cannot measure: sleep quality, sleep apnoea, depression, burnout and chronic fatigue syndrome. These causes are recognised in a conversation with a doctor.
- If fatigue comes with weight loss, night sweats, enlarged lymph nodes, breathlessness or fever, this is not a matter for a self-paid test. It is a matter for a doctor.
Fatigue is the most common reason someone decides to have a blood test. The good news: a small, well-chosen set of tests explains most of the cases that blood can explain at all. The bad news: some causes are not visible in blood, and any honest provider must tell you so.
First round: seven tests that explain the most
The order is not random. It follows how common the causes are in adults who come in because of fatigue.
| Test | What it looks for | What to watch for |
|---|---|---|
| Ferritin + hs-CRP | Iron stores; CRP tells whether they are falsely raised. | Below 30 µg/L means empty stores; with inflammation the threshold is 70 µg/L (WHO 2020). |
| Full blood count (FBC) | Anaemia, red cell size (MCV), inflammation. | Low MCV → iron; high MCV → B12, folate or alcohol. |
| TSH | Thyroid function. | If it is off, fT4 and anti-TPO follow; stop biotin 48 h before. |
| Vitamin B12 (and folate) | A deficiency that tires you and affects concentration. | In the grey zone of 150–300 pmol/L homocysteine helps; more common in vegans and on metformin. |
| Glucose and HbA1c | Diabetes and prediabetes. | HbA1c is not reliable with iron deficiency — then fasting glucose applies. |
| Vitamin D | A deficiency that is common in winter in Central Europe. | The link with fatigue is weaker than people say; makes sense with risk factors. |
| Liver and kidneys (ALT, GGT, creatinine, eGFR) | Silent organ causes. | 48–72 h without hard training and 24 h without alcohol, otherwise the values are falsely raised. |
Already have these results? Upload your report to Blood Lab 360 and get every value explained. If ferritin is low and the report shows no reason for iron loss, guidelines also recommend a coeliac test (tTG-IgA). In men and post-menopausal women they also recommend a check of the digestive tract.
Second round: when the first is normal
- Coeliac disease (tTG-IgA). With iron deficiency or long-lasting fatigue, guidelines look for it actively (NICE NG20, BSG 2021). The test is valid only on a normal gluten-containing diet.
- Thyroid antibodies (anti-TPO). Make sense with a borderline TSH, not a normal one. Otherwise they flag many people without consequence.
- Cortisol at 8–9 a.m. A screen for adrenal hormone deficiency. It is not a measure of “stress”, and “adrenal fatigue” is not a diagnosis.
- Testosterone (men). With a drop in libido and muscle mass. Morning draw, confirmed with a second measurement, with LH alongside.
- Infections. With long-lasting fatigue after an illness, liver tests and serology sometimes come into play. A doctor decides that, not a catalogue.
What blood can't tell you
This is the most important part of the article, because this is where most unnecessary tests are sold.
Blood cannot measure sleep quality (apnoea is found with a sleep study), depression and anxiety, burnout, chronic fatigue syndrome or overload. There is no recognised blood test for any of these conditions — not even a “stress hormone profile”.
If you have been sleeping five hours a night, have small children or are in a demanding period, your report will most likely be normal. That is not wasted money if it answers the question “is anything wrong”. But it is wasted if you expect it to explain something blood cannot see.
When to see a doctor instead of testing
- Fatigue with unintended weight loss, night sweats or fever.
- Enlarged lymph nodes, bruising without reason, bleeding.
- Breathlessness on little exertion, or chest pain.
- Fatigue that came on suddenly and is getting worse fast.
In these cases the right address is your GP. Some of the needed investigations (ultrasound, a sleep study, referrals) are beyond the reach of a self-paid blood test.
When should I have the blood drawn
In the morning between 7 and 10 a.m., fasting (12 hours). Not during an infection and not for two weeks after it — CRP and ferritin are misleading then. No hard training or alcohol the day before. If you take supplements with biotin, stop them 48 hours before. Detailed preparation guide.
Frequently asked questions
What is the single most telling test for fatigue?
Ferritin together with CRP. Iron deficiency is the most common cause blood can show. CRP prevents a falsely reassuring result with inflammation.
Ferritin is low but the blood count is normal — can that be the cause?
Yes. Iron stores empty before haemoglobin falls, and fatigue often appears already then. The deficiency threshold is below 30 µg/L; with inflammation 70 µg/L applies.
Should I measure cortisol if I am exhausted?
Morning cortisol is a screen for adrenal hormone deficiency, not a measure of stress. “Adrenal fatigue” is not a medical diagnosis and no cortisol profile confirms it.
All my results are normal. What now?
That is useful information: organic causes that blood can see are unlikely. The next steps are sleep (possible apnoea), mood, workload and physical fitness. Talk about this with your GP.
How long until I feel better if iron is the cause?
The first changes often come in 4–6 weeks, and the stores refill in 3 months. Re-test after three months, not sooner.
Sources: WHO 2020 — ferritin and CRP · NICE NG20 and BSG 2021 — coeliac disease with iron deficiency and fatigue · BSG 2021 — iron deficiency in men and after menopause · ATA/ETA — thyroid · BSH 2014 — B12 and the grey zone · ADA 2024 — glucose and HbA1c · Endocrine Society 2016 — morning cortisol · FDA 2019 — biotin and immunoassays.
