Feeling drained is so common that most people stop mentioning it. They assume it is age, or work, or sleep, and they are often right. But a meaningful share of persistent tiredness — the kind that does not lift after a good week — has a cause that shows up plainly on a routine blood test, and stays invisible until somebody orders one.
Iron, and why it is missed in the people who have it
Low iron is one of the most common nutritional deficiencies worldwide, and it can drain energy and concentration well before it becomes anaemia. It is also frequently missed because the symptoms are unremarkable: tiredness, breathlessness on stairs, cold hands, difficulty concentrating. Heavy periods, a vegetarian diet, gut conditions and regular blood donation all raise the odds.
B12, and the group most at risk
B12 deficiency produces fatigue and fogginess and, if it goes on long enough, nerve symptoms such as pins and needles. Absorption falls with age, so older adults are more exposed even on a good diet. So are people on long-term metformin or acid-suppressing medication, and anyone eating little or no animal food. It is cheap to measure and straightforward to correct.
The thyroid, which imitates everything
An underactive thyroid slows everything down: energy, mood, digestion, thinking. It is common, easy to test, and its symptoms are so general that people put them down to getting older. If tiredness comes with weight gain, feeling cold, dry skin or low mood, it belongs on the list of things to rule out.
Vitamin D and honest expectations
Deficiency is genuinely widespread in places with long winters and indoor work, and correcting it matters for bone and muscle health. What the evidence does not support is treating it as a general cure for tiredness in people whose levels are already adequate. Supplementing above what you need has no established benefit, and very high doses carry their own risks.
The diet part that is not about supplements
Most people who eat enough variety do not need pills for any of this. Protein at each meal, some legumes and leafy greens, oily fish where it is eaten, and not skipping meals covers most of the ground. Supplements are for a measured gap, not for a general sense that something is missing.
What to ask for, and when
Fatigue that persists for more than a few weeks despite reasonable sleep deserves a conversation and usually a simple panel: full blood count, ferritin, B12, thyroid function. Weight loss you did not intend, night sweats, breathlessness at rest, or blood where it should not be are reasons to be seen sooner rather than later.
The point of testing is not to find something wrong. It is to stop guessing about something that a single vial of blood can answer.