Why Am I Always Tired? A Doctor's Guide to Fatigue Investigation and the Blood Tests That Find Answers
Persistent tiredness is one of the most common reasons people visit a doctor, and one of the most frequently dismissed. If you are always tired despite adequate sleep, a targeted blood test can identify treatable causes in the majority of cases. The most common culprits are iron deficiency, vitamin D deficiency, underactive thyroid, vitamin B12 deficiency, and undiagnosed diabetes. All are detectable with a simple blood draw and all are treatable. At The Online GP by The Wellness, same-day fatigue blood test appointments are available at our Marylebone clinic, with results in 24 to 48 hours interpreted by a GMC-registered doctor who explains what your numbers mean and what to do about them.
Book a Fatigue Blood Test - WhatsApp | Email: team@thewellnesslondon.com | Call: 020 3951 3429
Why You Should Not Accept Tiredness as Normal
There is a widespread cultural tendency to normalise fatigue. "Everyone's tired." "It's just stress." "You're getting older." "Welcome to London." These responses, however well-intentioned, can delay the identification of genuinely treatable conditions that are silently draining your energy.
The truth is that persistent, unexplained fatigue is not normal. It is a symptom, and symptoms have causes. Many of those causes are remarkably common, remarkably simple to diagnose, and remarkably responsive to treatment.
Consider the prevalence of the most common causes. Iron deficiency affects approximately 1 in 4 women of reproductive age globally (WHO, 2024). Vitamin D deficiency affects approximately 1 in 5 UK adults, with higher rates during winter months and in people with darker skin tones (NICE, 2024). Hypothyroidism (underactive thyroid) affects approximately 1 in 20 people in the UK, with many cases undiagnosed for years (British Thyroid Foundation, 2025). Vitamin B12 deficiency is increasingly common, particularly among vegetarians, vegans, and older adults whose absorption declines with age. Pre-diabetes affects an estimated 1 in 3 UK adults, many of whom are unaware.
A single blood test appointment can screen for all of these. If one or more is positive, treatment can begin immediately and the improvement in energy levels is often dramatic.
The Fatigue Blood Panel: What We Test and Why
At The Online GP by The Wellness, our doctors select blood tests based on your specific symptoms, medical history, and risk factors. A comprehensive fatigue investigation typically includes the following markers.
Full blood count (FBC). The foundation of any fatigue investigation. Measures red blood cells, white blood cells, haemoglobin, and platelets. Identifies anaemia (low haemoglobin), which directly reduces the oxygen-carrying capacity of your blood and causes tiredness, breathlessness, and pallor. Also identifies raised white cells that may indicate infection or inflammation.
Iron studies (ferritin, serum iron, TIBC). Ferritin is the most sensitive marker of iron stores. Critically, ferritin can be low enough to cause fatigue and brain fog even when haemoglobin is still technically normal. This means you can be iron-depleted and symptomatic without meeting the formal definition of anaemia. Many patients are told their blood count is "fine" when their ferritin is actually suboptimal. Your doctor at The Online GP by The Wellness checks ferritin specifically and interprets it in context.
Thyroid function (TSH, free T3, free T4). An underactive thyroid slows metabolism across every system in the body, causing fatigue, weight gain, cold sensitivity, dry skin, hair thinning, constipation, low mood, and brain fog. TSH alone can miss early or subclinical thyroid dysfunction, which is why we test free T3 and free T4 as well. Thyroid antibodies can be added if autoimmune thyroid disease is suspected.
Vitamin D. Deficiency is extremely common in the UK due to limited sunlight, particularly from October to March. Symptoms include fatigue, muscle weakness, bone pain, low mood, and increased susceptibility to infections. Supplementation is straightforward and often transformative.
Vitamin B12 and folate. B12 deficiency causes fatigue, weakness, numbness or tingling in the hands and feet, cognitive difficulties, and mood changes. It is common in vegetarians and vegans (B12 is found primarily in animal products), older adults, and people taking certain medications (metformin, proton pump inhibitors). Folate deficiency compounds the problem.
HbA1c and fasting glucose. Screen for diabetes and pre-diabetes. Undiagnosed Type 2 diabetes causes fatigue, increased thirst, frequent urination, and blurred vision. The fatigue results from the body's inability to use glucose effectively for energy. Pre-diabetes is an earlier stage where intervention can prevent progression.
Liver and kidney function. Liver disease and kidney impairment can both cause fatigue. These markers are routinely included in a comprehensive panel and can identify issues that are otherwise silent in early stages.
Inflammatory markers (CRP, ESR). Elevated inflammation can cause fatigue and may point to underlying autoimmune conditions, chronic infections, or other inflammatory processes.
Calcium. Both high and low calcium levels can cause fatigue, along with other symptoms including muscle cramps, bone pain, and mood changes.
Your doctor may also recommend additional tests based on your presentation, including testosterone (men), female hormones (women), coeliac screening (if there are digestive symptoms), or cortisol (if adrenal insufficiency is suspected).
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What Happens When We Find the Cause
This is where doctor-led interpretation makes the critical difference. A traffic-light report from an online testing service might flag your ferritin as "amber" or your vitamin D as "red," but it cannot tell you what dose of supplement you need, how long treatment should last, when to recheck, or whether additional investigation is warranted.
At The Online GP by The Wellness, your doctor calls you to discuss every result. If iron deficiency is identified, your doctor prescribes the appropriate iron supplement at the correct dose, advises on timing and absorption (taken on an empty stomach with vitamin C, not with tea or coffee), and schedules a recheck in 8 to 12 weeks. If thyroid dysfunction is identified, your doctor can initiate levothyroxine treatment or refer to an endocrinologist. If vitamin D is low, your doctor prescribes a loading dose followed by maintenance, tailored to the severity of your deficiency. If diabetes or pre-diabetes is identified, your doctor discusses lifestyle modifications and, if appropriate, medication.
Often, addressing one or two deficiencies produces a noticeable improvement in energy within weeks. Patients frequently tell us they had forgotten what normal energy feels like.
What If Blood Tests Are Normal?
Normal blood tests are not a dead end. They are a starting point that rules out the most common medical causes. If your comprehensive blood panel returns entirely normal, your doctor will discuss other possibilities.
Sleep quality. You may be getting enough hours but not enough restorative sleep. Sleep apnoea (causing repeated nighttime oxygen drops) is common and underdiagnosed, particularly in men and those who are overweight. Insomnia, restless legs syndrome, and screen-related sleep disruption are other factors. A referral for sleep assessment may be appropriate.
Mental health. Depression, anxiety, and chronic stress are among the most common causes of fatigue. These are medical conditions, not personal failings, and they are treatable. If your doctor suspects a mental health component, they can discuss therapy options, medication if appropriate, and specialist referral.
Hormonal factors. Perimenopause in women and low testosterone in men can cause fatigue that standard blood panels may not capture. Specific hormone testing (which we offer) can investigate this further.
Lifestyle factors. Overtraining, poor nutrition, excessive caffeine dependency (masking rather than treating fatigue), chronic dehydration, and sedentary behaviour all contribute to tiredness. Your doctor can provide evidence-based lifestyle advice.
Chronic fatigue syndrome (CFS/ME). If fatigue persists for more than 3 months despite normal investigations and is accompanied by post-exertional malaise, cognitive difficulties, and sleep that does not refresh, CFS/ME may be considered. Diagnosis requires exclusion of other causes and is made clinically by an experienced doctor.
Discuss Your Fatigue With a Doctor - WhatsApp | Email: team@thewellnesslondon.com | Call: 020 3951 3429
Frequently Asked Questions
What blood tests should I have for tiredness? FBC, iron studies, thyroid function, vitamin D, B12, folate, HbA1c, liver and kidney function, inflammatory markers, and calcium as a minimum. Additional tests based on your symptoms.
What is the most common cause of fatigue on blood tests? Iron deficiency, vitamin D deficiency, and underactive thyroid are the three most commonly identified treatable causes.
Can tiredness be something serious? Usually not, but fatigue can be an early sign of diabetes, heart disease, kidney disease, or other conditions. Investigation is important to catch the rare serious cause early.
Should I see a doctor about being tired? Yes, if it is persistent (more than 2 to 4 weeks), unexplained, affecting your daily life, or accompanied by other symptoms.
How quickly can I get tested? Same-day appointments at our Marylebone clinic. Results in 24 to 48 hours. Doctor calls to discuss.
What if my blood tests are normal? Not a dead end. Your doctor will investigate sleep quality, mental health, hormonal factors, lifestyle, and other possibilities.
Can I combine a fatigue blood test with other investigations? Yes. Thyroid ultrasound, hormone panels, and ultrasound scanning can all be added in the same visit for a comprehensive assessment.
The Online GP by The Wellness provides private doctor-led healthcare from our Marylebone clinic. Contact us on WhatsApp at +44 7961 280835, email team@thewellnesslondon.com, or call 020 3951 3429.
References
WHO. Iron deficiency anaemia: global prevalence. Updated 2024.
NICE. Vitamin D deficiency in adults. Updated 2024.
British Thyroid Foundation. Hypothyroidism prevalence. Updated 2025.
Diabetes UK. Diabetes and pre-diabetes prevalence 2025.
NICE. Tiredness/fatigue in adults: assessment. CKS. Updated 2025.
