'Your labs are normal' but you still feel awful: fatigue, pain, inflammation and the gut–immune connection

How 'normal' ranges are actually made

Most laboratory reference ranges are built by testing a group of people considered healthy and taking the middle 95 per cent of their results (Jones & Barker, 2008). That has two consequences that are rarely explained to patients. First, by definition, 5 per cent of healthy people will fall outside the range. Second, and more relevant here, being inside the range tells you that you look like most of the reference population — not that the level is right for you, or that it hasn't fallen from where it used to be.

Ranges also differ between laboratories and countries, and some are built from populations that may not be especially healthy. Your own trend over time is often more informative than a single snapshot against a population range.

A normal result means you resemble most people in a reference group. It doesn't always mean that level is optimal for your body.

Ferritin: a clear example

Ferritin, the iron storage protein, is a good illustration. Many labs flag ferritin only when it falls very low, yet in a randomised trial of menstruating women with fatigue, low-normal ferritin (below 50 µg/L) and no anaemia, iron supplementation reduced fatigue more than placebo (Vaucher et al., 2012). A woman with heavy periods and a ferritin in the teens or twenties may be told her iron is 'fine' while still being meaningfully depleted.

Thyroid: where honest debate matters

Thyroid testing shows both sides of this argument. Some endocrinologists have argued that the upper limit of normal for TSH should be lower than it is (Wartofsky & Dickey, 2005); others argue the current range should stay as it is (Surks et al., 2005). And a meta-analysis found that treating people with mildly raised TSH with thyroid hormone did not, on average, improve symptoms or quality of life (Feller et al., 2018).

I include this deliberately, because functional medicine can overcorrect. 'Optimal ranges' are sometimes presented with more certainty than the evidence supports. My approach is to use them as a prompt for curiosity — look at the whole thyroid picture, including free T4, free T3 and antibodies, alongside symptoms and history — rather than as a reason to treat a number on its own.

What standard panels often leave out

When someone says their bloods were normal, my first question is which bloods. Basic panels often don't include the markers most relevant to fatigue, pain and inflammation. Depending on the person, it can be worth asking about:

• Ferritin and full iron studies, not just haemoglobin.
• Free T4, free T3 and thyroid antibodies, not just TSH.
• Active B12 or B12 with folate, vitamin D and magnesium.
• HbA1c and fasting insulin or glucose, for blood sugar regulation.
• High-sensitivity CRP, a more sensitive marker of low-grade inflammation than standard CRP (Ridker, 2003).
• Coeliac serology, if you have any gut symptoms, anaemia or a family history.
• Sex hormones in the context of your cycle or life stage, where symptoms suggest it.

The gut–immune connection

A large proportion of the body's immune tissue sits along the gut, where it has to tell the difference between food, friendly microbes and genuine threats every day (Vighi et al., 2008). The gut lining is a single layer of cells held together by tight junctions, and when that barrier is disrupted, more bacterial fragments and food components may reach the immune system than usual (Fasano, 2011).

One of those fragments, lipopolysaccharide from certain bacteria, has been linked in animal and human research with low-grade 'metabolic' inflammation (Cani et al., 2007). And inflammation itself changes how we feel: inflammatory signals act on the brain to produce what researchers call sickness behaviour — fatigue, low mood, aching, poor concentration and withdrawal (Dantzer et al., 2008). If you have ever felt flattened by a cold, you know the feeling. A chronic, low-level version is a plausible contributor to some unexplained symptoms.

Now the honest limits. 'Leaky gut' is a real biological phenomenon, but measuring it reliably in clinic is difficult, and its role as a cause rather than a consequence of disease is still debated (Camilleri, 2019). It is a useful perspective, not a proven root cause for everyone.

The gut and immune system are deeply connected — but 'it's your gut' is a hypothesis to explore, not a diagnosis to assume.

When pain persists and the tests are clear

Persistent pain with normal scans and bloods is one of the most demoralising experiences I see, because it is so easily heard as 'it's in your head'. One well-established explanation is central sensitisation: after long periods of pain, inflammation or stress, the nervous system can become more sensitive, amplifying signals so that pain is felt more strongly and more widely than the tissues alone would explain (Woolf, 2011).

That pain is real. It is produced by a measurable change in how the nervous system processes signals, and it is now recognised as part of many chronic conditions, including endometriosis, IBS and fibromyalgia. It also means that the most useful approach often combines several things — treating any ongoing inflammation, improving sleep, calming the stress response, graded movement and, where appropriate, pain specialist input — rather than searching endlessly for a single abnormal test.

Tests I'm cautious about

Because people who feel awful are understandably desperate for answers, they are also vulnerable to expensive tests that don't deliver. IgG food sensitivity panels are a good example: the European Academy of Allergy and Clinical Immunology advises against using them to diagnose food intolerance, because IgG antibodies to foods often reflect normal exposure rather than a problem (Stapel et al., 2008). Some stool and functional tests can add useful context in the right hands, but many are not well validated, and results should never replace a medical work-up.

It is also important not to miss the conditions that genuinely need medical diagnosis: sleep apnoea, depression, autoimmune disease, ME/CFS (NICE, 2021), long COVID, medication side effects, and many more. Functional medicine at its best widens the lens; it doesn't skip the basics.

What I actually do with clients

• Build a timeline: when did this start, what happened around then — infection, bereavement, a new medication, a pregnancy, a move?
• Ask for copies of results with the numbers, and look at trends over the last few years.
• Fill gaps in testing with your GP where they are justified, rather than ordering everything.
• Rebuild the foundations: protein at every meal, steady blood sugar, sleep, gentle consistent movement and fewer ultra-processed foods.
• Feed the gut diversity. Eating 30 or more different plant foods a week was associated with a more diverse gut microbiome in the American Gut Project (McDonald et al., 2018) — an association, not proof of cause, but a helpful target.
• Add fermented foods. In a 10-week trial, a diet high in fermented foods increased microbiome diversity and lowered several inflammatory markers (Wastyk et al., 2021). Full-fat organic kefir and yoghurt (or A2, sheep or goat versions), sauerkraut and kimchi are easy places to start.
• Review regularly and refer back to your doctor whenever something doesn't add up.

Before you act on any of this

This article is for education and general information only. It isn't medical advice, it can't diagnose or treat any condition, and it doesn't replace a consultation with your own doctor or a qualified practitioner who knows your history. Please don't start, stop or change any medication, hormone or supplement because of something you've read here — take your questions to the person responsible for your care.

References

• Camilleri, M. (2019). Leaky gut: mechanisms, measurement and clinical implications in humans. Gut, 68(8), 1516–1526.
• Cani, P.D., Amar, J., Iglesias, M.A., et al. (2007). Metabolic endotoxemia initiates obesity and insulin resistance. Diabetes, 56(7), 1761–1772.
• Dantzer, R., O'Connor, J.C., Freund, G.G., Johnson, R.W., & Kelley, K.W. (2008). From inflammation to sickness and depression: when the immune system subjugates the brain. Nature Reviews Neuroscience, 9(1), 46–56.
• Fasano, A. (2011). Zonulin and its regulation of intestinal barrier function: the biological door to inflammation, autoimmunity, and cancer. Physiological Reviews, 91(1), 151–175.
• Feller, M., Snel, M., Moutzouri, E., et al. (2018). Association of thyroid hormone therapy with quality of life and thyroid-related symptoms in patients with subclinical hypothyroidism: a systematic review and meta-analysis. JAMA, 320(13), 1349–1359.
• Jones, G., & Barker, A. (2008). Reference intervals. Clinical Biochemist Reviews, 29(Suppl 1), S93–S97.
• McDonald, D., Hyde, E., Debelius, J.W., et al. (2018). American Gut: an open platform for citizen science microbiome research. mSystems, 3(3), e00031-18.
• National Institute for Health and Care Excellence. (2021). Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management (NG206). London: NICE.
• Ridker, P.M. (2003). Clinical application of C-reactive protein for cardiovascular disease detection and prevention. Circulation, 107(3), 363–369.
• Stapel, S.O., Asero, R., Ballmer-Weber, B.K., et al. (2008). Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy, 63(7), 793–796.
• Surks, M.I., Goswami, G., & Daniels, G.H. (2005). The thyrotropin reference range should remain unchanged. Journal of Clinical Endocrinology & Metabolism, 90(9), 5489–5496.
• Vaucher, P., Druais, P.L., Waldvogel, S., & Favrat, B. (2012). Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ, 184(11), 1247–1254.
• Vighi, G., Marcucci, F., Sensi, L., Di Cara, G., & Frati, F. (2008). Allergy and the gastrointestinal system. Clinical & Experimental Immunology, 153(Suppl 1), 3–6.
• Woolf, C.J. (2011). Central sensitization: implications for the diagnosis and treatment of pain. Pain, 152(3 Suppl), S2–S15.
• Wartofsky, L., & Dickey, R.A. (2005). The evidence for a narrower thyrotropin reference range is compelling. Journal of Clinical Endocrinology & Metabolism, 90(9), 5483–5488.
• Wastyk, H.C., Fragiadakis, G.K., Perelman, D., et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153.