Why Am I Always Tired? 7 Blood Tests That Reveal the Answer

The hidden epidemic of modern fatigue

Persistent tiredness affects 1 in 5 UK adults. You sleep enough, you eat well, you exercise — yet by 3pm you are reaching for another coffee. Sound familiar?

The truth is, fatigue is rarely about willpower. More often, it is your body sending a signal that something measurable is off. And a simple blood test can reveal exactly what.

Here are seven biomarkers that most commonly explain unexplained fatigue — and what you can do about each one.

1. Ferritin — your iron reserves

Ferritin is the protein that stores iron in your cells. Even if your haemoglobin is normal, low ferritin means your reserves are depleted. The NHS considers ferritin below 13 ug/L as deficient in women, but many functional medicine practitioners flag anything below 30.

In UK Biobank data (500,000 adults), median ferritin for women aged 40-49 is just 42 ug/L — many are borderline without knowing it.

What to do: If ferritin is low, consider iron-rich foods (red meat, lentils, spinach) and vitamin C to enhance absorption. Your GP can prescribe ferrous sulphate if needed.

2. Vitamin D — the sunshine deficit

Living above the 50th parallel, the UK gets insufficient UVB radiation from October to March. Public Health England estimates 20-40% of adults are deficient. Vitamin D below 25 nmol/L is classified as deficient, below 50 as insufficient.

Symptoms of deficiency include fatigue, muscle weakness, low mood, and poor immune function — all of which overlap with general tiredness.

What to do: Supplement 1000-2000 IU daily, especially October to March. Test your levels to know where you stand.

3. Thyroid Function (TSH) — your metabolic thermostat

Your thyroid controls metabolism, energy, body temperature, and weight. An underactive thyroid (hypothyroidism) is one of the most common causes of fatigue — and it is frequently missed because symptoms develop slowly.

TSH above 4.2 mIU/L suggests hypothyroidism. But many people with TSH between 2.5 and 4.2 report fatigue — the so-called subclinical range.

What to do: If TSH is elevated, your GP can check Free T4 and thyroid antibodies. Treatment with levothyroxine is straightforward and life-changing.

4. Vitamin B12 — nerve fuel

B12 is essential for nerve function, red blood cell production, and DNA synthesis. Deficiency causes fatigue, brain fog, tingling, and even depression. It is particularly common in vegetarians, vegans, older adults, and people on metformin or PPIs.

What to do: If B12 is below 300 pmol/L, consider supplementation. Below 148 pmol/L is frank deficiency requiring treatment.

5. HbA1c — your blood sugar memory

HbA1c reflects your average blood sugar over 2-3 months. Even in the prediabetic range (42-47 mmol/mol), energy crashes and afternoon fatigue are common as your body struggles with insulin resistance.

What to do: Reduce refined carbohydrates, increase physical activity (150 min/week), and improve sleep. These lifestyle changes can reverse prediabetes completely.

6. CRP — hidden inflammation

C-reactive protein reveals chronic low-grade inflammation — the kind you cannot feel but which drains your energy. A CRP above 3 mg/L is associated with increased cardiovascular risk and systemic fatigue.

What to do: Address root causes — body composition, diet (reduce processed foods), sleep quality, and stress management. Omega-3 fatty acids (2g/day EPA+DHA) have been shown to reduce CRP.

7. Haemoglobin — oxygen transport

Haemoglobin carries oxygen from your lungs to every cell. Anaemia (Hb below 120 g/L in women, 130 g/L in men) is a classic cause of fatigue, breathlessness, and pale skin.

What to do: If haemoglobin is low, investigate the cause — iron deficiency, B12/folate deficiency, or chronic disease. Treatment depends on the underlying cause.

The complete picture

No single marker tells the whole story. Fatigue is often the result of multiple small deficiencies compounding — slightly low iron, borderline vitamin D, suboptimal B12, and mild inflammation all adding up.

That is why a comprehensive blood test panel — not just one or two markers — gives you the clearest picture of what your body needs.