Symptom Guide

Always Tired? What Blood Tests Can Tell You

Name: Always Tired? Blood Tests That Explain Constant Fatigue
Creator: Suman Konda
Published: 2026-07-05

Persistent fatigue is one of the most common complaints in general practice.¹ Most causes are identifiable with a simple blood test panel. Here's what to check and what results mean.

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

Blood tests to order for fatigue

If you are constantly tired despite adequate sleep, your doctor will typically order these tests to find the cause:

1. CBC (Complete Blood Count)

Checks for anaemia: the most common cause of fatigue worldwide. Low haemoglobin means your blood is carrying less oxygen to your muscles and brain. Even mild anaemia (Hb 10–11 g/dL) causes significant fatigue.

2. Thyroid Profile (TSH)

An underactive thyroid (hypothyroidism) is extremely common, especially in women over 30. It causes fatigue, weight gain, hair loss, constipation and feeling cold. TSH above 4.0 mIU/L suggests hypothyroidism. Treatment with levothyroxine often brings energy levels back to normal.

3. Vitamin B12

B12 deficiency causes profound fatigue, along with tingling in hands/feet and memory problems. Extremely common in people following vegetarian or vegan diets. B12 below 200 pg/mL is deficient and needs treatment with supplements or injections.

4. Vitamin D

Deficiency is common worldwide, particularly in people with limited sun exposure, darker skin or covered dress. Symptoms include fatigue, muscle aches, bone pain and low mood. Vitamin D below 20 ng/mL is deficient and highly treatable with supplements.

5. Ferritin (Iron Stores)

Even without full anaemia, low ferritin (iron stores below 30 ng/mL) causes fatigue. This is especially common in women with heavy periods. Ferritin can be low when haemoglobin still appears normal, which is why ferritin must be tested separately.

6. Fasting Blood Sugar / HbA1c

Uncontrolled or undiagnosed diabetes causes fatigue because cells can't use glucose properly. Many people with Type 2 diabetes don't know they have it. A fasting sugar or HbA1c screens for this.

7. Liver Function Test (LFT)

Liver disease causes fatigue, especially fatty liver (NAFLD), which is increasingly common. Elevated SGPT/ALT is a common finding in people with unexplained tiredness and metabolic syndrome.

What if all tests are normal?

If all routine blood tests are normal but fatigue persists, other causes to consider include: sleep apnea (interrupted breathing during sleep), depression or anxiety, overwork and burnout, chronic fatigue syndrome (ME/CFS), or medications causing drowsiness. Discuss these possibilities with your doctor if labs are unrevealing.

Questions to ask your doctor

Should I test all seven of these, or start with the most likely ones based on my situation?
My B12/D/ferritin is normal. What else should we check?
Is my fatigue possibly related to my diet, sleep quality or stress?

References

Sources cited on this page. PubMed links open the original abstract.

Nijrolder I, van der Windt DA, de Vries H, van der Horst HE. Diagnoses during follow-up of patients presenting with fatigue in primary care. CMAJ. 2009;181(10):683–687. PMID 19858240 · doi:10.1503/cmaj.090647

A structured approach to investigating fatigue

Fatigue is among the most common complaints in primary care, and the differential diagnosis spans nearly every organ system. A structured clinical approach avoids both over-investigation (ordering dozens of tests without clear clinical indication) and under-investigation (dismissing fatigue as "stress" without excluding treatable causes).

First-line blood tests for unexplained fatigue of more than 4 weeks, without an obvious explanation, should include:

FBC: Anaemia – from any cause – is a highly treatable and commonly missed cause of fatigue. The MCV guides the type of anaemia (iron deficiency, B12/folate deficiency, haemolysis).
TSH: Hypothyroidism is the single most important condition to exclude in any fatigued patient – it is common (particularly in women over 40), treatable, and its symptoms (fatigue, depression, weight gain, cold intolerance, constipation) closely match the generic "tired all the time" presentation.
Fasting glucose or HbA1c: Undiagnosed or poorly controlled type 2 diabetes causes fatigue, particularly through osmotic polyuria and dehydration. Reactive hypoglycaemia in pre-diabetic states also causes post-meal fatigue.
U&E (urea and electrolytes) and creatinine: Uraemia, hyponatraemia, and hypokalaemia all cause fatigue and are easily missed without routine electrolyte testing.
LFTs: Chronic liver disease – including asymptomatic fatty liver disease – causes fatigue. A raised GGT in particular may signal alcohol use or NAFLD.
ESR and CRP: Non-specific inflammatory markers. A raised ESR in a fatigued patient without other obvious cause warrants a wider screen for occult infection, inflammatory disease, or malignancy.
Vitamin D: Severe deficiency (25-OH vitamin D below 25 nmol/L) causes fatigue, myalgia, and low mood. While milder insufficiency is not clearly causative of fatigue, supplementation in deficient individuals often improves energy levels.
B12 and folate: Both cause fatigue and neurological symptoms if deficient. B12 deficiency is particularly common in vegans, strict vegetarians, elderly people, and those with autoimmune gastritis (pernicious anaemia).

Myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS)

ME/CFS is characterised by disabling fatigue lasting more than 6 months that is not improved by rest and is worsened by physical or cognitive exertion (post-exertional malaise – PEM). Associated features include unrefreshing sleep, cognitive impairment ("brain fog"), and orthostatic intolerance. ME/CFS is a clinical diagnosis – all standard blood tests are normal (which is necessary to make the diagnosis, not evidence against it). The NICE guideline (NG206, 2021) recommends against graded exercise therapy (GET) and recommends energy management (pacing) as the primary approach.