Why am I so tired all the time?

What is usually behind it

Feeling tired all the time is one of the most common reasons people see a GP, and most of the time the cause is ordinary and findable. The usual suspects are poor or short sleep, low iron stores, low vitamin D in the winter months, low B12 (especially on a plant-based diet or with a gut condition), an underactive thyroid, ongoing stress, and recovery from a virus.

See your GP if tiredness has lasted more than a few weeks, is getting worse, comes with weight change, breathlessness, pale skin, heavy periods, low mood, or pain, or if you simply cannot explain it. Tiredness that persists deserves a blood test, not a supplement.

What a blood test can show. A full blood count and ferritin will show whether iron stores are low (in the UK a ferritin under 30 is the usual threshold for deficiency). Vitamin D, B12 and folate are often tested together, and thyroid function (TSH) is the other standard check. If you have results already, the free interpreter reads them against the UK ranges: Free blood test result interpreter.

What people in this position most often look at. Iron, where ferritin is low: iron contributes to the reduction of tiredness and fatigue. Vitamin D from October to March: UK guidance is that everyone considers a daily supplement over winter. Vitamin B12: B12 contributes to the reduction of tiredness and fatigue and to normal energy-yielding metabolism. Magnesium: magnesium contributes to the reduction of tiredness and fatigue. None of these replaces the blood test; they are what the result points to when it points anywhere.

How long things take. Iron, B12 and vitamin D each have their own timeline.

Next: Supplements for energy and tiredness · Ask Nutri365 to read your results together.

Written by Nutri Tailor from the Health Reference Library. Last checked 6 September 2026. General information, not personal advice.