Deficiencies & testing

When deficiency symptoms need a blood test — and what tests can and cannot tell you

Written by the VitaWhat editorial team · Medically reviewed by Steve Davis · Last reviewed 24 July 2026 · Editorial policy

Tiredness, hair changes and poor concentration can point to a nutrient shortfall — but they have dozens of other causes. Here is when to get tested, and how to read the results sensibly.

Many of the symptoms linked to nutrient deficiencies — fatigue, low mood, poor concentration, brittle nails, hair thinning — are among the most common reasons people feel below par. The catch is that these symptoms are non-specific: the same tiredness could come from low iron, low vitamin B12, an underactive thyroid, poor sleep, stress or dozens of other causes. That is exactly why a symptom is a reason to investigate, not a diagnosis in itself.

Why you should not self-diagnose from a symptom list

Reference pages (including ours) list the signs of each deficiency to help you understand the topic. But because those signs overlap so heavily, matching your symptoms to a list will often point to the wrong cause. Two risks follow from guessing:

You treat the wrong thing — taking iron for tiredness that was actually caused by low B12 or poor sleep, and losing time before the real cause is found.
You mask a problem — high-dose folic acid, for instance, can hide the blood signs of B12 deficiency while nerve damage quietly progresses.

When a blood test is worth it

Consider seeing a GP for testing if:

Symptoms are persistent (weeks rather than days) or getting worse.
You have a reason to be at higher risk — a restrictive or vegan diet, pregnancy, heavy periods, a gut condition affecting absorption (such as coeliac or Crohn's disease), weight-loss surgery, or older age.
Symptoms are affecting daily life — breathlessness, a racing heart, significant hair loss, or numbness and tingling.

Numbness or pins-and-needles deserve prompt attention, because they can signal B12 deficiency, where early treatment matters for preventing lasting nerve damage.

What common tests actually measure

Ferritin reflects your iron stores and is the usual first test for suspected iron deficiency. Important caveat: ferritin also rises with infection or inflammation, so a "normal" result during an illness can hide low iron.
Vitamin B12 is usually measured as total B12, which does not always reflect how much is available to your cells. If the picture is unclear, a doctor may add tests such as MMA or active-B12.
Vitamin D is measured as 25-hydroxyvitamin D — a reliable marker of your status, and genuinely useful in winter or for people with little sun exposure.
Folate can be checked in blood; results are affected by recent diet.

What tests cannot tell you

Blood levels do not always equal tissue levels. Magnesium is the classic example — most is stored in bone and cells, so a normal blood magnesium can coexist with a real shortfall.
A snapshot is not a trend. One result reflects a single moment; recent diet, illness or even the time of day can shift it.
"Normal range" is not the same as "optimal for you." Reference ranges describe most of the population, and results near a boundary need interpreting alongside your symptoms and history — which is a doctor's job, not a calculator's.

The single most useful principle: test before you treat. Starting high-dose supplements *before* testing can distort the very result that would have told you what was wrong.

What to do with the results

If a test confirms a deficiency, treating it properly means two things: correcting the shortfall (through diet, supplements or, for B12, sometimes injections) and looking for the underlying cause. A deficiency is often a clue — for example, unexplained iron deficiency in an adult always warrants a look at why it happened, not just an iron tablet.

If tests come back normal but you still feel unwell, that is useful information too: it points the search somewhere other than nutrition, such as sleep, thyroid function, mood or other conditions your GP can explore.

The bottom line

Use nutrient reference material to understand what *might* be going on and to have a better-informed conversation with a clinician. Let a blood test, interpreted by a professional alongside your history, tell you what is *actually* going on — and resist the urge to supplement your way through symptoms before you know their cause.