Vitamin deficiency tests: what to ask your GP and how to read results
A blood test takes the guesswork out: whether you need a supplement, at what dose and when to stop. But testing everything is pointless. This page covers which tests are useful and how to understand the results.
Ferritin levels explained
Ferritin shows your iron stores. Ferritin normal range by WHO and BSG cut-offs, why lab ranges differ, low ferritin and iron deficiency symptoms, when to see a GP.
9 min readLow magnesium: symptoms and foods
Magnesium deficiency symptoms, why a blood test for low magnesium says little, who is at risk and which magnesium-rich foods give the most per serving (NIH data).
9 min readVitamin D test and levels
Vitamin D levels explained: 25(OH)D in nmol/L and ng/mL (×2.5), who needs a vitamin D test, deficiency symptoms and why lab ranges differ. NHS and NIH guidance.
9 min read
How this section works
For each deficiency we answer three questions: which symptoms point to it, which test confirms it and what to do with the result. Thresholds come from the NHS, WHO and NIH ODS, but your GP has the final word on your results.
Which test to ask about
Pick what bothers you to see which result to discuss with your GP
What bothers you?
What to check
Full blood count and ferritin
The NHS usually starts with a full blood count (FBC) to check for iron deficiency anaemia. Ferritin shows iron stores but rises with inflammation, so it's read alongside other results.
How to read it: reference ranges depend on the lab
What to check
Ferritin and a GP visit
Iron deficiency is one cause of temporary hair loss. Other causes (stress, illness, weight loss, hormones) won't show up on a vitamin test.
If you take biotin, tell your GP: it distorts several blood tests.
What to check
25(OH)D, the vitamin D test
It's the only test that reflects vitamin D stores. UK guidelines don't recommend routine testing for people without symptoms: a daily 10 mcg in autumn and winter is enough.
How to read it: under 25 nmol/L deficient, 25–50 insufficient, over 50 sufficient (UK guidance)
What to check
Vitamin B12
Vegans get almost no B12 from food, and absorption falls with age and with metformin. A test helps decide whether you need an ongoing supplement.
What to check
A blood test says little
Almost all magnesium sits in bones and cells, so the serum level doesn't reflect stores. The decision rests on diet and symptoms, together with your GP.
Vitamin deficiency tests: what to ask for
Start from your symptoms, not from a long vitamin panel on a private lab’s price list. For tiredness, breathlessness and pale skin, the NHS usually starts with a full blood count; ferritin shows your iron stores. See ferritin levels and low ferritin. For vitamin D, read vitamin D test and levels; for magnesium, magnesium deficiency symptoms.
Topics in this section
| What to check | Test | How to read it | Guide |
|---|---|---|---|
| Iron stores | ferritin, full blood count | cut-offs vary by lab; ferritin rises with inflammation | ferritin |
| Vitamin D | 25(OH)D | under 25 nmol/L deficient, over 50 sufficient (UK) | vitamin D levels |
| Vitamin B12 | serum B12 | read alongside symptoms | vitamin B12 |
| Magnesium | serum magnesium | a normal result doesn’t rule out a shortfall | magnesium deficiency |
Units vary between reports: 1 ng/mL of vitamin D = 2.5 nmol/L. Other conversions are in our supplements A–Z, and you can convert vitamin D right here:
Vitamin D: convert ng/mL to nmol/L and back
Pick the units on your report and enter the number
50 nmol/L
1 ng/mL = 2.5 nmol/L
20 on your report = 50 nmol/L
UK guidance: under 25 nmol/L is deficient, 25–50 nmol/L insufficient, over 50 nmol/L sufficient.
UK thresholds are set in nmol/L, while US sources often use ng/mL. Your GP reads the result alongside your symptoms.
What can distort a result
- Biotin distorts thyroid, troponin and several other tests. If you take it, tell your GP.
- Inflammation and infection push ferritin up, which can hide iron deficiency.
- A supplement taken on the day of the test counts too: ask your GP whether to pause it.
- Compare results in the same units and, ideally, from the same lab.
UK guidance doesn’t recommend routine vitamin D testing for people without symptoms. The NHS simply advises 10 mcg a day in autumn and winter.
Before you buy a supplement based on a test
- Discuss the result with your GP: a low level can point to an illness, and not only to a gap in your diet.
- Ask about the dose and when to retest.
- Don’t take iron “just in case”: too much is harmful, and high doses are dangerous for children.
- With kidney or liver disease, doses should come from your doctor.
How to choose on iHerb after a test
- Start from the dose your GP recommended and find a product with a convenient strength.
- Check the units on the label: mcg, mg or IU.
- Compare it with what’s already in your multivitamin.
- Note your start date so you can retest on time.
Bottom line: what to choose
Buttons lead to selections of the right form on iHerb. These are affiliate links; your price doesn't change.
FAQ
Which vitamin deficiency tests should I ask my GP for?
It depends on your symptoms. For tiredness and pale skin, a full blood count and ferritin. For bone pain or muscle weakness, vitamin D (25(OH)D). For vegans, people over 60 and anyone on metformin, B12. Broad “vitamin panels” without symptoms are rarely useful.
What is a normal vitamin D level in the UK?
UK guidance treats under 25 nmol/L as deficient, 25–50 nmol/L as insufficient and over 50 nmol/L as sufficient. US sources often use ng/mL: multiply by 2.5 to get nmol/L.
What does low ferritin mean?
Ferritin reflects your iron stores, so a low level points to deficiency. The cut-off depends on the lab. Ferritin rises with inflammation: the WHO suggests that in adults with infection or inflammation, a level under 70 µg/L may still indicate iron deficiency.
Should I stop supplements before a blood test?
Tell your GP about everything you take. Biotin matters most: it can distort thyroid, troponin and other tests. Agree with your GP when to pause it.
Can a blood test show low magnesium?
Not reliably. Almost all magnesium sits in bones and cells, and the body keeps the blood level steady for as long as it can. A normal result doesn’t rule out a shortfall, so diet and symptoms count too.
Sources
- NHS. Iron deficiency anaemia
- NHS South West London. Vitamin D guidelines for adults and children
- WHO. Use of ferritin concentrations to assess iron status (eLENA)
- NIH ODS. Magnesium – Fact Sheet for Health Professionals
- NIH ODS. Vitamin B12 – Fact Sheet for Health Professionals
- NIH ODS. Biotin – Fact Sheet for Health Professionals
What to check
Situation and the test that tells you something
- Tired and pale
- full blood count and ferritin
- Little sunlight
- 25(OH)D: under 25 nmol/L is deficient (UK)
- Vegan, metformin, 60+
- vitamin B12
- Suspected low magnesium
- a blood test reflects stores poorly
- Taking biotin
- tell your GP and the lab
Your GP reads results alongside symptoms: reference ranges differ between labs.