You sleep eight hours, eat reasonably healthily and still you hit the wall at three in the afternoon. Your colleague adds a double espresso on top; you know that does not fix the problem. Fatigue is one of the most common reasons to visit a GP, and at the same time one of the hardest to interpret. The cause rarely lies in a single thing.
My stance: with months of unexplained fatigue, guessing is a waste of your time. A targeted blood test gives data instead of guesswork. Below we go through seven values directly related to energy, with for each one what it is, why it affects your energy and when action makes sense. First the overview.
| Blood value | What it measures | Point of attention with fatigue |
|---|---|---|
| Ferritin | Iron stores | Complaints possible below 30 ug/L, even without anaemia |
| TSH | Thyroid function | Outside roughly 0.4-4.0 mU/L: have a doctor explain it |
| Vitamin D | Vitamin D status | Often low in winter in the Netherlands |
| Vitamin B12 | B12 status | Risk with vegetarian diet, older age, acid reducers |
| Haemoglobin | Oxygen transport | Low = anaemia; always look for the cause |
| CRP | Inflammation marker | Raised = immune activity, which costs energy |
| HbA1c | Average blood sugar (2-3 months) | 42-47 mmol/mol falls in the prediabetes range |
1. Ferritin: your iron stores
Ferritin is the storage form of iron. It is the first value to drop with iron deficiency, before your haemoglobin falls. A low ferritin is one of the most common causes of chronic fatigue, especially in women. Many labs use a lower limit around 15 ug/L, but complaints can occur below 30 ug/L. Women with heavy periods, vegetarians and intensive athletes are at extra risk. The tricky part is that you can carry a low ferritin for months before your blood count officially deviates.
- Constant fatigue, even after a good night
- Concentration problems and forgetfulness
- Hair loss and brittle nails
- Cold hands and feet, sometimes restless legs
2. TSH: your thyroid function
Your thyroid is the thermostat of your metabolism. TSH is the value used to assess its function. A high TSH points to an underactive thyroid (hypothyroidism): your metabolism slows and you feel exhausted. When in doubt, free T4 is often measured too. Thyroid complaints are more common in women and after age 40, and are regularly missed because they develop gradually.
- Extreme fatigue and listlessness
- Weight gain without a clear cause
- Feeling cold, dry skin and hair loss
- Slow heartbeat and constipation
3. Vitamin D: the sunlight vitamin
The Netherlands is not a land of abundant sunlight. Between October and March your skin makes hardly any vitamin D. The Health Council (Gezondheidsraad) therefore advises extra vitamin D for groups such as older people, people with darker skin and those who spend little time outdoors. A deficiency plays a role in muscle function, the immune system and mood, and can cause fatigue and low mood, especially in winter.
- You mainly work indoors
- You have a darker skin tone
- You eat little oily fish or fortified products
- You wear covering clothing
4. Vitamin B12: essential for your nervous system
Vitamin B12 is essential for making red blood cells and for your nervous system. A deficiency develops slowly, sometimes over years, and the complaints are vague enough to be missed for a long time. Risk groups are vegetarians and vegans (B12 is mainly in animal products), older people and those who use acid reducers.
- Fatigue and a general sense of weakness
- Tingling or numbness in hands and feet
- Memory problems and loss of concentration
- A sore, red tongue
5. Haemoglobin: oxygen through your body
Haemoglobin (Hb) transports oxygen from your lungs to your tissues. If it is too low (anaemia), muscles and organs get too little oxygen: you feel tired, weak and short of breath on exertion. Anaemia is not a diagnosis in itself but a consequence, for example of iron deficiency, B12 deficiency or blood loss. So with a low Hb, always look for the underlying cause.
- Men: roughly 8.5 to 11.0 mmol/L
- Women: roughly 7.5 to 10.0 mmol/L
6. CRP: a signal of inflammation
CRP is an inflammation marker; your liver makes more of it during inflammation. Chronic low-grade inflammation is an underestimated cause of fatigue: you have no fever, but your body is constantly switched on. That costs energy and can show up as fatigue, muscle pain and malaise.
- Infections, viral or bacterial
- Auto-immune diseases or chronic bowel inflammation
- Excess weight (fat tissue produces inflammatory substances)
- Long-term stress and lack of sleep
7. HbA1c: your average blood sugar
HbA1c reflects your average blood sugar over 2-3 months and is the standard for detecting prediabetes and diabetes. Both a high and a strongly fluctuating blood sugar can cause fatigue: with insulin resistance, glucose enters your cells less well, so they get too little fuel. The Diabetes Fonds places an HbA1c of 42-47 mmol/mol in the prediabetes range, a phase that is often still reversible.
- Energy dips after meals
- Frequent thirst and more frequent urination
- Difficulty losing weight despite effort
- Blurred vision and slow wound healing
What can you do yourself?
Fatigue is rarely a matter of simply sleeping more. If you have been tired for weeks to months without explanation, a targeted blood test is a logical first step. A broad test such as the complete metabolic panel covers a large part of these values; if the thyroid is suspected, a thyroid function test is more targeted.
- Test purposefully: a broad energy panel gives a fuller picture than a single value
- Take the grey zone seriously: technically normal but not optimal can already cause complaints
- Discuss deviations with your doctor: a blood test is a starting point, not a final diagnosis
- Combine with lifestyle: sleep, movement and nutrition stay relevant, even with normal values
Frequently asked questions
When should I see a GP about fatigue?
If you have been persistently tired for more than two weeks without a clear cause, get in touch. Certainly with additional complaints such as unexplained weight loss, fever or night sweats.
Does my health insurance cover a blood test?
A blood test on referral from your GP is usually covered from the basic package, after your deductible. If you order one yourself without a referral, you pay the costs yourself.
How quickly do I get the result?
At most laboratories you have the result within 2 to 5 working days. Some values, such as a complete blood count, are often available sooner.
Can I have several deficiencies at once?
Yes, that happens regularly. An iron deficiency often goes together with a low vitamin D or B12. That is why a broad panel is more useful than a single value.
Do I need to fast for this blood test?
For most values it is not strictly necessary. For glucose and HbA1c, fasting (8 to 12 hours, water is allowed) can improve interpretation. You receive instructions in advance with your order.
The bottom line
With unexplained fatigue, a targeted blood test gives you something concrete to act on. Every blood test result at Vitalcheck includes a professional review by a BIG-registered doctor. A blood value is not a diagnosis: always discuss treatment decisions with your GP. Also read how poor sleep and your blood values connect, or the pillar on burnout and your body.
Sources
- NHG guideline Anaemia. Dutch College of General Practitioners.
- Gezondheidsraad (Health Council of the Netherlands). Advice on vitamin D supplementation. Accessed 2026.
- Diabetes Fonds (Dutch Diabetes Foundation). Prediabetes and HbA1c. Accessed 2026.
- Thuisarts.nl / NHG. I am tired. Accessed 2026.
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