The five biomarkers most commonly missed by GPs when investigating unexplained fatigue — ferritin, Free T3, vitamin D, cortisol, and insulin resistance — and how a comprehensive blood test finds what standard NHS testing misses.
Blood Test for Fatigue UK — What Your GP Won't Check
If you are experiencing unexplained fatigue, your GP will typically check haemoglobin and TSH. But fatigue is often driven by markers they don't test — ferritin, Free T3, vitamin D, cortisol, and insulin resistance. A comprehensive panel testing all of these can find what a standard NHS blood test misses.
Fatigue is one of the most common reasons people visit their GP, and one of the most poorly investigated. The standard approach is a full blood count and a thyroid check. If your haemoglobin is above 120 and your TSH is between 0.4 and 4.0, you will be told you are fine. But "fine" by laboratory reference range standards is not the same as optimal, and several of the most common causes of fatigue are not included in standard NHS testing at all.
Ferritin — the marker GPs miss most often. Ferritin is your body's iron storage protein. The laboratory reference range typically starts at 10-13, meaning a ferritin of 14 would be reported as normal. But for an active person, especially a menstruating woman, functional depletion begins below 50. A ferritin of 14 is technically within range but functionally empty. This is the single most common cause of unexplained fatigue in women of reproductive age, and it is routinely missed because the number falls within the lab range. A GP will not flag it. A comprehensive panel with optimal ranges will.
Free T3 — the active thyroid hormone nobody checks. Your GP tests TSH, and maybe Free T4. If both are normal, your thyroid is declared fine. But T4 is the inactive storage form. It must be converted to T3 in your tissues to actually drive your metabolism. Some people convert poorly due to stress, calorie restriction, selenium deficiency, or chronic inflammation. Their TSH and T4 look normal but their Free T3 is low, meaning their cells are running on a throttled metabolic rate. This directly causes fatigue, brain fog, and cold intolerance. Most GPs never test Free T3.
Vitamin D — deficient in most of the UK population. The NHS considers vitamin D sufficient above 25 nmol/L, but optimal is 75-150. A level of 38 would be reported as adequate by your GP but is genuinely deficient by functional standards. Low vitamin D causes fatigue, low mood, muscle weakness, and impaired immune function. It is one of the easiest things to fix with supplementation but only if you know your level.
Cortisol — the stress hormone driving exhaustion. Chronically elevated cortisol from stress, poor sleep, or overtraining causes a particular kind of fatigue — wired but tired, difficulty waking, energy crashes in the afternoon. Standard NHS testing does not include morning cortisol unless an endocrine condition is specifically suspected. A comprehensive panel checks it as standard.
Insulin and HOMA-IR — the metabolic fatigue nobody talks about. Insulin resistance causes fatigue after meals, energy crashes, brain fog, and difficulty losing weight. Your fasting glucose can be completely normal while your insulin is elevated, meaning your pancreas is working overtime to keep your blood sugar in range. A GP checking only glucose or HbA1c will miss this entirely. Fasting insulin and HOMA-IR catch insulin resistance years before it progresses to pre-diabetes.
One of our customers, Nadia, was told by her GP twice that her blood tests were normal. She was experiencing fatigue, headaches, and mood swings. Our Ultimate panel found functionally depleted ferritin, low Free T3 despite normal TSH, elevated cortisol, low progesterone, deficient vitamin D, and borderline magnesium — six interconnected findings that explained all three of her symptoms. Her GP had only checked five markers. We tested 110.
If you have been told your bloods are normal but you still feel exhausted, the problem is almost certainly that the right things were not tested. A comprehensive blood test looking at ferritin, Free T3, vitamin D, cortisol, and insulin alongside a full panel of 110 biomarkers will either find the cause or give you genuine peace of mind that everything truly is optimal.
See inside a real TrueVitals report.