Why Am I Always Tired? What Blood Tests Can Tell You — Private Fatigue Investigation in London

Persistent tiredness is one of the most common reasons people visit a doctor — and one of the most frustrating, because the NHS response is often limited. A 10-minute appointment, a basic blood test checking only haemoglobin and TSH, results reported as "normal," and advice to get more sleep. Meanwhile, you are still exhausted.

The problem is rarely that nothing is wrong. The problem is that not enough has been tested.

The Online GP by The Wellness offers comprehensive fatigue investigation blood panels from £99, with same-day blood draws and results within 24-48 hours. Our doctors do not just check whether your levels are "within range" — they look at where you sit within that range and what your results mean for you as an individual.

Tired of being tired? WhatsApp us now to arrange same-day blood tests. Or email team@thewellnesslondon.com.

The Most Common Causes of Persistent Fatigue

Iron Deficiency — The Silent Energy Drain

Iron deficiency is the world's most common nutritional deficiency, affecting an estimated 3-4 million people in the UK. You can be iron-depleted without being anaemic — your haemoglobin may be technically normal, but your iron stores (measured by ferritin) are too low to support optimal energy.

This is where NHS testing often falls short. Many labs report ferritin as "normal" if it is above 12-15 mcg/L. But research consistently shows that fatigue symptoms can occur when ferritin drops below 30 mcg/L, and many patients feel significantly better when their ferritin is above 50. Knowing your actual ferritin number — not just whether it is flagged as abnormal — is essential.

Women of reproductive age are particularly vulnerable due to menstrual blood loss, but men can also develop iron deficiency from poor dietary intake, gut absorption issues, or occult blood loss that needs investigation.

Thyroid Dysfunction

Your thyroid gland controls your metabolic rate — effectively, how fast your body's engine runs. An underactive thyroid (hypothyroidism) slows everything down: energy, metabolism, heart rate, digestion, and brain function. Around 2% of the UK population has hypothyroidism, and it is roughly 10 times more common in women.

Symptoms overlap significantly with general fatigue, which is why blood testing is essential. They include persistent tiredness regardless of sleep, unexplained weight gain, feeling cold when others are comfortable, dry skin and brittle hair, constipation, low mood and difficulty concentrating, and heavier or irregular periods in women.

A TSH test alone — which is what many NHS GPs order — can miss early or subclinical thyroid problems. A comprehensive thyroid panel includes TSH, free T4, free T3, and thyroid antibodies (TPO and TgAb). Thyroid antibodies can detect autoimmune thyroid disease (Hashimoto's) years before TSH becomes abnormal.

Vitamin D Deficiency

An estimated 1 in 5 UK adults has low vitamin D, rising to 1 in 3 during winter months. This is unsurprising given the UK's latitude and limited sunlight between October and March. Fatigue is one of the primary symptoms, along with muscle weakness, bone pain, low mood, and susceptibility to infections.

Vitamin D is not routinely tested on the NHS unless you have specific risk factors. Private testing is simple, inexpensive, and gives you a clear number. If your level is low, supplementation is straightforward and inexpensive, and most people notice an improvement within 4-8 weeks.

Vitamin B12 and Folate Deficiency

B12 is essential for nerve function, red blood cell production, and energy metabolism. Deficiency causes fatigue, brain fog, tingling or numbness in hands and feet, mood changes, and a particular type of anaemia (megaloblastic anaemia). Vegetarians, vegans, older adults, and people on certain medications (particularly metformin and proton pump inhibitors) are at higher risk.

Folate deficiency causes similar symptoms and often coexists with B12 deficiency. Both are easily detected by blood test and corrected with supplementation or, in the case of B12, injections.

Diabetes and Pre-Diabetes

Persistently elevated blood sugar — whether diagnosed diabetes or the pre-diabetic state — causes fatigue because your cells cannot effectively use glucose for energy. Other symptoms include increased thirst, frequent urination, blurred vision, and slow wound healing.

An HbA1c blood test measures your average blood sugar over the previous 2-3 months. It can detect both diabetes and pre-diabetes, often years before symptoms become obvious. Early detection means early intervention, which can prevent or delay the progression to full diabetes.

Do not guess. Test. WhatsApp us to book a same-day fatigue blood panel.

Hormonal Imbalance

In women, perimenopause and menopause are among the most common — and most under-recognised — causes of debilitating fatigue. Fluctuating oestrogen and progesterone affect sleep quality, mood, and energy. Many women in their 40s and early 50s are told their tiredness is "stress" when it is actually hormonal.

In men, declining testosterone causes fatigue, low motivation, reduced muscle mass, and poor concentration. Testosterone deficiency is more common than most men realise, particularly after age 40.

Hormonal blood panels can identify these imbalances and guide treatment.

Other Causes

Persistent fatigue can also result from chronic infections (such as Epstein-Barr virus reactivation), coeliac disease (present in roughly 1 in 100 people, many undiagnosed), kidney or liver dysfunction, chronic inflammation (measured by CRP and ESR), sleep disorders including obstructive sleep apnoea, and mental health conditions including depression and anxiety (which Blog 7 covers in detail).

A comprehensive blood panel, combined with a proper clinical history, can identify or rule out the majority of these causes in a single appointment.

Our Fatigue Blood Panel

Our doctors have designed a fatigue investigation panel specifically to catch the conditions that basic NHS testing misses. This includes full blood count, iron studies (serum iron, ferritin, transferrin saturation, TIBC), comprehensive thyroid panel (TSH, free T4, free T3, thyroid antibodies), vitamin D, vitamin B12 and folate, HbA1c (diabetes screening), kidney function, liver function, CRP (inflammation), and a metabolic panel.

Additional tests can be added based on your symptoms, including hormonal profiles (oestrogen, progesterone, testosterone, SHBG, cortisol), coeliac screen, and full lipid profile.

Fatigue blood panels start from £99. Results are available within 24-48 hours, and a doctor reviews every result with you personally — explaining not just whether levels are "normal" but what they mean for your health and what steps to take next.

For patients who want the most comprehensive picture, our executive health assessment from £495 includes extensive blood work alongside cardiovascular assessment, body composition analysis, and an extended GP consultation.

You deserve to know why you are exhausted. WhatsApp us now to arrange same-day blood testing.

📧 Email team@thewellnesslondon.com

📞 Call 020 3951 3429

📍 10 Portman Square, Marylebone, London W1H 6AZ

This article was written by the medical team at The Online GP by The Wellness. Last updated: February 2026. Persistent fatigue lasting more than 4 weeks warrants medical investigation. If you are experiencing sudden, severe fatigue alongside other symptoms such as chest pain, shortness of breath, or unexplained weight loss, please seek urgent medical attention.