You're sleeping enough. You're eating reasonably well. You're not doing anything obviously wrong — and yet you're exhausted. Every day. The kind of tired that doesn't shift with an early night or a strong coffee.
If this sounds familiar, you're not imagining it and you're not just "getting older." In most cases, persistent fatigue has an underlying cause — and a private blood test is one of the fastest, most reliable ways to find it.
Here are the six things I see most commonly in blood work that explain why people feel chronically tired.
1. Low ferritin (iron stores)
Ferritin is your body's stored iron — and it's the single most important marker for iron-related fatigue. The problem is that ferritin is frequently missed on standard NHS blood tests, which often only measure serum iron or haemoglobin. You can have a completely normal haemoglobin reading and still have ferritin levels so low that your body is struggling to function properly.
Low ferritin causes fatigue that feels heavy and relentless — the kind where even small tasks feel effortful. It's also one of the most common causes of hair loss, brain fog, breathlessness, and restless legs. It's particularly common in women with heavy or irregular periods, and in perimenopause.
The fix is straightforward once you know the problem — but you can't fix what you haven't found.
2. Vitamin D deficiency
Vitamin D deficiency affects the majority of UK adults, particularly between October and March when sunlight is limited. It's directly linked to fatigue, low mood, poor immunity, muscle weakness, and bone pain — a cluster of symptoms that are easy to dismiss or attribute to other causes.
What makes vitamin D deficiency so easy to miss is that it develops gradually and its symptoms are vague. Many people spend months or years feeling below par without realising their vitamin D is critically low. A simple blood test gives you a precise number — and supplementing at the right dose can make a noticeable difference within weeks.
3. Underactive thyroid (hypothyroidism)
Your thyroid gland controls your metabolism — the rate at which your body produces and uses energy. When it's underactive, everything slows down. The fatigue associated with hypothyroidism is distinctive: heavy, persistent, and accompanied by weight gain, feeling cold all the time, dry skin, hair loss, constipation, and low mood.
Thyroid problems are significantly more common in women than men and become more prevalent with age. A full thyroid panel — TSH, Free T3, and Free T4 — gives a complete picture of thyroid function. TSH alone, which is all that's typically tested on the NHS, can miss early or subclinical thyroid dysfunction.
4. B12 deficiency
B12 plays a critical role in energy production, nerve function, and the formation of red blood cells. Deficiency causes fatigue, brain fog, difficulty concentrating, mood disturbances, and in more severe cases, tingling in the hands and feet.
B12 deficiency is more common than most people realise — particularly in people over 40, vegetarians and vegans, and anyone taking certain long-term medications including metformin or proton pump inhibitors. The standard B12 test measures total B12 in the blood, but active B12 — the biologically available form that your cells can actually use — is a more sensitive and clinically meaningful marker. We test both.
5. Hormonal imbalances
Hormones regulate almost every system in your body, and when they're out of balance, fatigue is almost always one of the first symptoms. In women, low oestrogen, low progesterone, and low testosterone — all of which decline during perimenopause — are closely linked to exhaustion, poor sleep, and low energy that doesn't respond to rest.
In men, low testosterone is one of the most underdiagnosed causes of fatigue, low motivation, reduced strength, and poor concentration. Cortisol imbalances — linked to chronic stress and adrenal dysfunction — can also drive persistent tiredness, particularly the pattern of feeling wired at night and exhausted in the morning.
A comprehensive hormone panel gives you a clear picture of what's happening and what needs addressing.
6. Anaemia
Anaemia — where your blood doesn't carry enough oxygen to your tissues — is one of the most well-known causes of fatigue, but it comes in several forms that require different approaches. Iron-deficiency anaemia is the most common, but anaemia can also be caused by B12 or folate deficiency, chronic disease, or other underlying conditions.
A full blood count alongside iron studies and B12 identifies which type of anaemia is present — and that distinction matters, because treating iron-deficiency anaemia with B12 supplementation won't help, and vice versa.
Why not just go to the GP?
You can, and you should rule out serious illness through your GP. But NHS blood testing is often limited to a narrow set of markers, and getting an appointment to discuss fatigue — let alone getting a comprehensive panel requested — can take weeks. Many of the markers most relevant to fatigue, including active B12, ferritin, full thyroid panels, and comprehensive hormone profiles, are not routinely tested.
Private blood testing gets you more markers, faster results, and a clinician who has time to actually talk you through what they mean.
What does a blood test for fatigue involve at Boost & Glow?
At Boost & Glow, Lucy carries out a venous blood draw — the same method used in hospitals and GP surgeries — either at our clinic in Epping, Essex, or at your home across Essex, Hertfordshire, and London. We offer panels specifically designed to investigate fatigue, covering ferritin, full iron studies, vitamin D, active B12, folate, full thyroid function, a full blood count, and hormone levels where relevant.
Results are returned promptly, with clear guidance on what they show and what to do next. If your results indicate something that needs medical follow-up, we'll advise you on the right next steps.
Stop guessing. Start knowing.
Persistent tiredness is not something you have to accept. In the majority of cases there's an identifiable, treatable cause — and a blood test is the fastest way to find it.
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