On this page
Start here
- [Low ferritin checker](/low-ferritin) explains ferritin cut-offs (including the AGA's 45 ng/mL threshold), why ferritin can look normal during inflammation, and what usually happens next.
- [TIBC and iron panel calculator](/tibc-blood-test) turns serum iron and TIBC into transferrin saturation and unsaturated iron-binding capacity, and shows the classic patterns of iron deficiency, anaemia of inflammation and iron overload.
Which vitamin or iron tool do you need?
Vitamin levels, iron studies and haemoglobin are among the most commonly checked blood tests, and they are reported in different units around the world. This section covers:
- [Vitamin D converter](/vitamin-d-ng-ml-to-nmol-l) – 25-hydroxyvitamin D, ng/mL × 2.496 = nmol/L
- [Vitamin B12 converter](/vitamin-b12-pg-ml-to-pmol-l) – pg/mL × 0.7378 = pmol/L
- [Folate converter](/folate-ng-ml-to-nmol-l) – ng/mL × 2.266 = nmol/L
- [Ferritin converter](/ferritin-ng-ml-to-pmol-l) – ng/mL equals µg/L; × 2.247 for pmol/L
- [Serum iron converter](/iron-mcg-dl-to-umol-l) – µg/dL × 0.1791 = µmol/L
- [Transferrin saturation calculator](/transferrin-saturation-calculator) – iron ÷ TIBC × 100
- [Haemoglobin converter](/hemoglobin-g-dl-to-g-l) – g/dL × 10 = g/L
Key values at a glance
| Test | Commonly used threshold | Meaning |
|---|---|---|
| Vitamin D (25-OH) | below 12 ng/mL (30 nmol/L) | Deficiency (US National Academy of Medicine) |
| Vitamin D (25-OH) | 20 ng/mL (50 nmol/L) or more | Adequate for bone health for most people |
| Vitamin B12 | below about 200 pg/mL (148 pmol/L) | Possible deficiency; borderline zone up to about 300 pg/mL |
| Serum folate | below about 3–4 ng/mL (7–10 nmol/L) | Possible deficiency |
| Ferritin | below about 30 ng/mL (µg/L) | Iron deficiency likely in adults |
| Transferrin saturation | below 20% / above 45% | Low iron availability / possible iron overload |
| Haemoglobin | below 13 g/dL (men), 12 g/dL (women) | Anaemia (WHO thresholds) |
Putting the results together
Iron deficiency
Low ferritin is the most specific sign. Transferrin saturation is usually low, and haemoglobin falls later. Ferritin also rises with inflammation, so a "normal" ferritin does not always exclude iron deficiency in people with chronic illness – transferrin saturation helps here.
B12 and folate
Both are needed to make red blood cells, and deficiency of either can cause large red cells (macrocytic anaemia). B12 deficiency can also cause nerve damage, sometimes before anaemia appears. Borderline B12 results are often clarified with methylmalonic acid or holotranscobalamin tests. Folate should not be given alone to someone who may be B12 deficient without checking B12, because it can mask the anaemia while nerve damage progresses.
Vitamin D
Vitamin D is measured as 25-hydroxyvitamin D. Thresholds differ between organisations, and there is debate about optimal levels beyond bone health. Large randomised trials have not shown that supplementing people with adequate levels prevents cancer or heart disease.
When to seek medical advice
See a clinician for symptoms of anaemia (tiredness, breathlessness, pale skin), numbness or tingling, memory problems, bone pain or muscle weakness, or if your results are outside the reference range. Men and post-menopausal women with iron deficiency should be checked for a source of bleeding in the gut. Do not take iron supplements unless deficiency is confirmed.
Frequently asked questions
Are ng/mL and µg/L the same for ferritin?
Yes, they are numerically identical.
What vitamin D level should I aim for?
Most guidelines consider 20 ng/mL (50 nmol/L) adequate for bone health. Some organisations suggest higher levels for certain groups.
Can I have iron deficiency with normal haemoglobin?
Yes. Iron stores fall before haemoglobin does, and iron deficiency without anaemia can still cause fatigue.