Why am I always tired? The 6 blood markers behind constant fatigue

"Why am I always tired?" is one of the most common questions asked in GP surgeries across the UK — and one of the hardest to answer in a ten-minute appointment. Sleep, stress and busy lives explain a lot of it. But when tiredness hangs around for weeks despite decent sleep, the answer is often hiding in your blood — and the good news is that the usual suspects are a short, well-understood list.

Here are the six blood markers behind most persistent fatigue, what each one does, and how to find out where yours sit.

1. Ferritin — your iron store

Ferritin is the protein your body uses to store iron, and low ferritin is one of the most common treatable causes of tiredness in the UK — especially in women under 40, where heavy periods can drain iron stores faster than diet replaces them. You can be short of iron long before you become anaemic: the fatigue arrives first, the anaemia later.

Feels like: heavy-limbed exhaustion, breathlessness on stairs, pale skin, sometimes hair shedding.

Check it with: the Iron and Ferritin Blood Test, or as part of a wider panel below.

2. Vitamin D — the winter one

Your skin makes vitamin D from sunlight, and for roughly half the year the UK sun is too weak to help. Around one in five UK adults runs low, and the symptoms are exactly the vague ones people put down to modern life: tiredness, low energy, aching muscles, a flat mood. The NHS does not routinely test it, which is why so many people never find out.

Feels like: tired-all-the-time with aches, worse from about October to March.

Check it with: the Vitamin D, B12 and Folate Blood Test — which also covers the next marker on this list.

3. Vitamin B12 — the nerve and blood-cell builder

B12 keeps your nerves and red blood cells healthy, and your body cannot make it — it has to come from food (mainly meat, fish, eggs and dairy) or supplements. Levels fall slowly, so the fatigue creeps up over months. Vegans, vegetarians, people on long-term acid-reflux medication and adults over 60 are the most likely to run low.

Feels like: exhaustion plus pins and needles, brain fog or a sore tongue.

Check it with: the same Vitamin D, B12 and Folate Blood Test.

4. TSH — your thyroid's dial

TSH (thyroid-stimulating hormone) is the signal your brain sends to your thyroid, and it is the first thing the NHS checks when tiredness walks through the door. An underactive thyroid slows everything down: energy, metabolism, concentration, even your heart rate. It often drifts for years before other symptoms make it obvious, and it is far more common in women.

Feels like: exhaustion with weight gain, feeling the cold, dry skin, thinning hair, low mood.

Check it with: the Thyroid Blood Test with Antibodies, which looks at the full picture rather than TSH alone.

5. HbA1c — your three-month blood sugar

HbA1c shows your average blood sugar over roughly the last three months — it is the same marker used to screen for type 2 diabetes and prediabetes. Blood sugar that runs high (or swings hard) is a classic cause of energy crashes, afternoon slumps and waking unrefreshed, and in the UK a large share of people with prediabetes have no idea.

Feels like: energy crashes after meals, thirst, needing the loo more, slow-healing cuts.

Check it with: the HbA1c and Fasting Glucose Blood Test.

6. Testosterone — not just a gym number

In men, testosterone drives energy, muscle, motivation and mood as much as anything in the gym. Levels decline gradually with age, but low testosterone at any age commonly shows up first as flat, unexplained tiredness with low drive — often years before anyone thinks to test it. (Women make testosterone too, in much smaller amounts, and it matters for energy there as well.)

Feels like: low energy plus low motivation, reduced libido, slower recovery from exercise.

Check it with: the Total Testosterone Blood Test.

One draw instead of six

You do not need six separate tests. The Men's Health Blood Test and Women's Health Blood Test cover all of the markers above — plus a full blood count, which rules anaemia in or out directly — from a single sample, finger-prick or venous. Every result comes back in your Console with the laboratory's own reference range beside it and each marker explained in plain English, and if you retest later, Trends charts the direction your numbers are moving rather than leaving you comparing printouts.

When tiredness is not a blood-test question

Some symptoms should go straight to a doctor rather than a test kit. See urgent medical help if tiredness comes with chest pain, severe breathlessness, fainting, sudden weakness, confusion, unexplained weight loss or black stools. And if your results ever flag something outside range, the right next step is your GP — with your lab report in hand.

Common questions

Can a blood test really find why I'm tired? Often, yes — low ferritin, vitamin D, B12 and thyroid problems are all common, all measurable and all treatable. But blood tests cannot measure sleep quality, stress or mood, which cause just as much tiredness. Test to rule the physical causes in or out, not as the whole answer.

Do I need to fast before a fatigue blood test? Not for most of these markers. Fasting matters mainly for glucose — each product page states exactly what its laboratory asks for.

How quickly do results come back? Turnaround is shown on every product page, and you can watch your sample's live status in Console from the moment the lab receives it.

This article is for education only and is not medical advice, diagnosis or treatment. Reference ranges vary between laboratories; your DragonDX report always shows the range your laboratory supplied. If you are worried about your symptoms, speak to your GP.