The metabolic and hormonal causes of persistent low energy that standard NHS blood testing misses, including functional iron depletion, thyroid conversion issues, cortisol dysregulation, and insulin resistance.
If you have persistent low energy despite adequate sleep, the most common biochemical causes are functional iron depletion (ferritin below 50, often reported as "normal"), poor thyroid hormone conversion (low Free T3 with normal TSH), vitamin D deficiency, cortisol dysregulation, insulin resistance, and chronic low-grade inflammation. Standard NHS testing checks almost none of these at the thresholds that actually matter. TrueVitals tests all of them as part of a 110-biomarker panel with a 60-page personalised report for £349.
Low energy is different from sleepiness. Sleepiness is resolved by sleep. Low energy is a persistent state of physical and mental depletion that does not improve with rest. It affects your ability to exercise, concentrate, socialise, and function at work. It is the number one reason people seek private blood testing in the UK, and it is the symptom most likely to be dismissed by a GP with "your bloods are fine."
The reason GPs miss the cause is not incompetence. It is that the testing protocol for investigating fatigue in primary care is extremely limited. A GP investigating tiredness will typically request a full blood count and TSH. If your haemoglobin is above 120 g/L and your TSH is between 0.4 and 4.0, you are told your blood tests are normal. But "normal" and "optimal" are not the same thing, and several of the most important markers for energy are never tested at all.
Ferritin — the energy marker hiding in plain sight:
Ferritin is your body's iron storage protein. The laboratory reference range typically starts at 10-13 ng/mL. A ferritin of 14 would be reported as normal. But ferritin below 50 is associated with fatigue, exercise intolerance, and reduced cognitive function even in the absence of anaemia. For actively training individuals, optimal ferritin is 50-150. The gap between "not anaemic" and "optimal iron stores" is enormous, and millions of people in the UK — particularly menstruating women, vegetarians, and athletes — sit in this gap, being told they are fine when they are functionally depleted.
One of our customers, Nadia, had a ferritin of 14. Her GP told her it was normal. Our report identified it as the primary driver of her fatigue and headaches, connected it to her training load and dietary patterns, and recommended iron bisglycinate at 25mg every other day with vitamin C. That single finding — missed twice by her GP — explained her primary symptom.
Free T3 — the metabolic throttle:
T3 is the active thyroid hormone that drives cellular energy production. Your GP tests TSH, which reflects pituitary signalling, not cellular metabolism. Many people have a normal TSH and adequate Free T4 but poor conversion of T4 to T3 in peripheral tissues. This is common in calorie restriction, chronic stress, selenium deficiency, and systemic inflammation. The result is a cellular metabolic rate running at 70-80% capacity. You feel exhausted, cold, sluggish, and mentally slow despite "normal" thyroid tests. Free T3 reveals this. Your GP does not test it.
Vitamin D — the UK's silent epidemic:
The majority of UK adults are vitamin D deficient for at least half the year. At 38 nmol/L you are deficient. Your GP may consider anything above 25 as adequate. But optimal for energy, mood, and immune function is 75-150 nmol/L. The difference between 38 and 100 is the difference between dragging yourself through the day and feeling genuinely vital. Vitamin D is cheap to supplement and easy to optimise, but only if you know your level.
Cortisol — when stress becomes exhaustion:
The cortisol pattern in chronic stress evolves over time. Initially cortisol rises, creating the "wired but tired" feeling — anxious energy paired with physical exhaustion. Over time, the adrenal response can flatten, producing low morning cortisol and a loss of the natural cortisol awakening response. This creates profound morning fatigue that improves slightly through the day. A single morning cortisol test gives limited information, but when interpreted alongside DHEA-S, Free T3, and the clinical picture, it reveals whether adrenal stress is contributing to low energy.
Insulin resistance — the energy crash cycle:
When cells become resistant to insulin, glucose delivery to cells becomes inefficient. The body compensates by producing more insulin, creating spikes and crashes in blood sugar that manifest as energy crashes, post-meal fatigue, brain fog, and cravings. Fasting glucose and HbA1c can remain normal for years while this pattern develops. Fasting insulin and HOMA-IR detect it early, allowing intervention through dietary changes before pre-diabetes develops.
Inflammation — the invisible energy drain:
Elevated hs-CRP in an otherwise healthy person suggests chronic low-grade inflammation from training stress, poor gut health, insufficient sleep, or dietary factors. Inflammation is metabolically expensive — your immune system is diverting energy from daily function to chronic immune activation. Reducing inflammation through targeted nutrition, sleep optimisation, and recovery management can produce a noticeable energy improvement within weeks.
B12 and folate — the methylation pair:
Both vitamins are essential for energy production at the cellular level. Suboptimal levels impair methylation, reduce neurotransmitter production, and slow DNA repair. Fatigue, brain fog, and mood changes are common at levels well above the deficiency threshold. Active B12 is a more sensitive marker than total B12 for detecting functional insufficiency.
A comprehensive blood test examining all of these markers together reveals the pattern behind low energy. It is rarely a single cause — more commonly it is 2-3 interconnected factors creating a cumulative drain on your body's resources. The TrueVitals 60-page personalised report identifies these connections and provides a targeted action plan.
See inside a real TrueVitals report →