Fatigue is one of the most common reasons patients visit a GP — and one of the most commonly missed. A standard NHS blood test checks a fraction of what's relevant. Here's what to actually ask for, and why.
Why Standard Blood Tests Miss the Cause
A typical NHS panel includes a full blood count, kidney function, liver function, and sometimes thyroid. That's a start — but it skips several tests that are frequently abnormal in fatigued patients: ferritin, vitamin D, B12, folate, morning cortisol, and sex hormones. A result "in the normal range" also doesn't mean it's optimal for you.
The Core Fatigue Blood Test Panel
Ferritin
Iron stores. The single most common cause of fatigue in women under 50. Target >50 µg/L for resolution of symptoms.
Full Blood Count
Checks haemoglobin and red cell indices. Low Hb confirms anaemia; MCV guides whether it's iron, B12 or folate.
TSH + Free T4
Hypothyroidism causes fatigue, weight gain, and brain fog. TSH alone can miss subclinical cases — always check free T4 too.
Vitamin D
Deficiency is widespread in the UK and directly associated with fatigue, low mood, and muscle weakness.
B12 & Folate
Both are needed for red cell maturation and nerve function. Deficiency causes profound, persistent tiredness.
HbA1c / Glucose
Undiagnosed type 2 diabetes or insulin resistance causes fatigue, especially post-meal crashes.
Morning Cortisol
Low cortisol (adrenal fatigue) causes profound tiredness worst in the mornings. Must be drawn before 9am.
Sex Hormones
Low testosterone (men and women) and oestrogen depletion around menopause cause significant energy loss.
Extended Panel — When the Core Tests Are Normal
If your core panel comes back unremarkable and fatigue persists, the next step is an extended screen:
| Test | What It Finds |
|---|---|
| CRP + ESR | Inflammation markers — raised in autoimmune conditions, infections, and inflammatory disease |
| Transferrin saturation | Confirms iron deficiency when ferritin is borderline |
| Free T3 | Active thyroid hormone — can be low even when TSH is normal |
| DHEA-S | Adrenal reserve marker, often depleted with chronic stress |
| Fasting insulin | Identifies insulin resistance before HbA1c becomes abnormal |
| Coeliac screen (tTG IgA) | Coeliac disease causes malabsorption, fatigue, and GI symptoms |
How to Interpret Your Results
Lab reference ranges are statistical norms — they tell you what's common, not what's optimal. Ferritin above 12 is "normal"; most functional medicine and sports medicine practitioners wouldn't treat until ferritin exceeds 50–70 µg/L. Similarly, TSH of 4.2 is technically normal but would explain profound fatigue in many patients. A GP who looks at the trend and your symptoms alongside the number will give you a better answer than one who reads "normal" and stops there.
Book a Fatigue Blood Test in Surbiton
Our comprehensive fatigue panel covers iron stores, thyroid, hormones, vitamins, and inflammation — reviewed by a GP with same-week results.
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