Why am I always tired? 10 causes a standard blood test can miss
Tired despite normal blood tests? Explore 10 contributors to fatigue and a functional medicine approach to nutrients, hormones, sleep and energy.
- Updated
- 4 min read
- By Charmaine Shepherd, IFM Certified Practitioner
Persistent fatigue deserves a fuller explanation. A basic blood panel may leave iron stores, nutrient status, hormonal changes, sleep problems and post-viral illness unexplored. In my clinic, we look at how these factors interact, review patterns in your results and build a plan around what your body needs to recover.
If getting through an ordinary day feels like hard work, being told your results are normal rarely feels like an answer.
Fatigue is one of the most common reasons people come to see me. Some have felt exhausted for years. Others can pinpoint an infection, a hormonal transition or a period of sustained pressure after which their energy never returned.
I want to understand that story. When does your energy dip? Do you wake refreshed? What happens after meals or exercise? Those details help us decide where to look.
What does the NHS say about tiredness?
The NHS recognises that persistent tiredness can be linked to sleep, medicines, hormonal changes, mental health and physical illness. Fatigue lasting several weeks or affecting everyday life warrants assessment.
That initial assessment matters. Functional medicine adds time to explore the connections, especially when the first round of investigations has not explained your symptoms.
Why can blood tests be normal when you’re exhausted?
A normal result tells us about the markers measured, at that point in time. It does not assess every aspect of energy production, nutrient absorption or restorative sleep.
I look at which tests were included, where the results sit within their ranges, and whether they have changed over time. We also consider factors that blood tests cannot capture, such as sleep apnoea or worsening symptoms after activity.
Read Why are my blood tests normal but I still feel unwell?
10 causes of tiredness that can be missed
| Contributor | Clues worth exploring | How we investigate |
|---|---|---|
| 1. Low iron stores | Heavy periods, hair shedding, restless legs, reduced stamina | Ferritin, blood count and wider iron studies where needed; inflammation can affect ferritin |
| 2. B12 or folate deficiency | Tingling, brain fog, restricted diet, digestive problems or relevant medicines | Dietary and medicine review, B12 and folate; additional markers when results are unclear |
| 3. Low vitamin D | Muscle weakness or aches, limited sunlight exposure | Vitamin D testing where indicated and an appropriate replacement plan |
| 4. Thyroid changes | Cold intolerance, constipation, dry skin, hair loss | TSH and free T4, with antibodies and sometimes free T3 to explore the wider picture |
| 5. Blood sugar regulation | Energy dips after meals, cravings, central weight gain | Meal history, HbA1c and glucose; fasting insulin or glucose monitoring where useful |
| 6. Sleep apnoea | Snoring, gasping, morning headaches, waking unrefreshed | A sleep assessment; a normal daytime oxygen reading does not exclude it |
| 7. Perimenopause | Cycle changes, night sweats, disrupted sleep, brain fog | Symptom timeline and cycle history; hormone metabolite testing when it adds useful context |
| 8. Digestive problems | Bloating, altered bowel habits, food reactions, recurring deficiencies | Coeliac screening, assessment of absorption, and targeted stool or small intestinal bacterial overgrowth (SIBO) testing |
| 9. Post-viral illness or long COVID | Fatigue starting after infection, delayed crashes after effort | Clinical assessment, screening for overlapping conditions and individual pacing |
| 10. Sustained stress and poor recovery | Feeling tired but wired, fragmented sleep, reduced resilience | Sleep and workload history, nervous system assessment and sometimes HRV biofeedback |
These are possibilities to investigate, rather than diagnoses made from symptoms alone. Medicine side effects, depression and other illnesses also deserve consideration.
How do I investigate tiredness?
I begin with your timeline and review previous results. We explore food intake, digestion, sleep, menstrual health, infections and the demands on your day.
Testing may include iron studies, B12, folate, vitamin D, thyroid markers, glucose regulation or inflammation. Where persistent fatigue raises questions about nutrient metabolism, I may consider a urine-based metabolic assessment such as Metabolomix. This provides indirect clues that we interpret with your history; it does not diagnose mitochondrial disease or prove the cause of fatigue.
For relevant hormonal symptoms, I often find urinary hormone metabolites useful for exploring hormone metabolism beyond a single blood measurement. Thyroid function still needs blood testing.
We then prioritise the findings: correcting deficiencies, supporting digestion, improving sleep, making meals more sustaining and helping your nervous system recover. HRV biofeedback can be useful for some people.
If activity causes a delayed worsening of symptoms, we work within your energy limits. Pushing through or following fixed increases in exercise can make post-exertional malaise worse.
See my fatigue and long COVID guide.
When to see your GP, or get urgent help
Persistent fatigue needs medical assessment, particularly with unexplained weight loss, fever, night sweats, swollen glands, breathlessness, heavy bleeding or excessive thirst and urination.
Call 999 for chest pain, severe breathlessness or immediate danger from thoughts of harming yourself. For emotional support, Samaritans is available on 116 123.
Common questions
Which blood tests should I ask for if I’m always tired?
A useful assessment may include a blood count, ferritin, B12, folate, thyroid function, HbA1c, kidney and liver markers, with vitamin D and coeliac screening where appropriate. The choice should reflect your symptoms and history.
Can low ferritin cause tiredness without anaemia?
Yes. Iron depletion can contribute to fatigue before haemoglobin falls. We also need to understand why your stores are low, including blood loss, intake or absorption.
Is being tired all the time a sign of perimenopause?
It can be, particularly with sleep disruption and cycle changes. I also check for overlapping contributors such as iron depletion and thyroid problems.
How long does it take to get your energy back?
That depends on what is driving the fatigue and how long it has been present. We track your daily function and recovery as well as test results, adjusting the pace to your capacity.
Tired of being tired? Book a free discovery call and let’s discuss what deserves a closer look.
Sources
Ferritin “normal” but still exhausted? How to read your iron results
Normal vs optimal: 5 blood test ranges explained
What happens at your first functional medicine appointment?
This article is for general information and isn't a substitute for personal medical advice. Please don't start or stop any medication without speaking to your GP or prescriber.