Low serum iron: what it means and what to do

A low serum iron means little iron is circulating in your blood right now. It does not always mean you are short of iron: it also falls with inflammation, and it changes a lot from day to day. Ferritin shows whether your store is really low.

What low serum iron means

Serum iron measures the iron circulating in your blood at that moment, not your store. It falls when iron is short, but also when the body holds iron back, for example during inflammation.

That is why a low number alone does not show you are deficient. If your ferritin is low too, your store is running down. If your ferritin is normal, something else is often behind it.

What causes low serum iron

Some empty your iron store, others only lower the iron in circulation.

• Heavy periods and pregnancy: In women before menopause they are the most common causes of low iron.
• Little meat on your plate: Iron from plants is absorbed less well. People who eat little or no meat more often have low iron.
• A lot of endurance sport: People who do a lot of endurance sport more often have low iron.
• Iron is not absorbed: For example with coeliac disease, after stomach surgery or with stomach-acid medicines taken for years.
• Blood loss: Frequent blood donation, or small hidden losses from the gut. In men and after menopause, doctors look for them first.
• Inflammation or infection: The body holds iron back in its store: serum iron falls even when the store is full.

Symptoms

Low serum iron on its own causes no symptoms. If there are any, they depend on how short of iron you really are:

• Tiredness that does not go away
• Less strength in sport
• Breathlessness on exertion
• More hair falling out than usual
• Restless legs in the evening

These symptoms have other causes too: ferritin and haemoglobin help tell whether iron is involved.

Low serum iron with normal ferritin

This happens often. During inflammation or an infection, the body holds iron back in its store and leaves less in circulation. So serum iron falls, while ferritin stays normal or rises.

If your CRP is high too, inflammation is usually more likely than a deficiency. But inflammation can also make ferritin look higher than it is: your doctor decides whether more tests are needed.

If your CRP is normal and you feel well, it is often just a day-to-day swing.

Which tests to read alongside it

These tests show whether iron is really short.

What your result means

The lower end of the scale, in µg/dL.

• Below your report’s limit, Usually below 33: Low. Little iron is circulating right now. On its own it does not show whether your store is empty. Check your ferritin. If it is low too, talk to your doctor.
• Within your report’s limits, Usually 33 to 193: Normal. Usually fine. But it does not show how big your store is: ferritin does. In the Rosa report you also see whether you are in your optimal range.

What to do if it is low

First find out whether iron is really short. Where to start:

• Check your ferritin: if it is low, your store is running down and your doctor looks for the cause.
• If your ferritin is normal, there is often nothing to do about iron. Talk to your doctor if you have symptoms or your CRP is high.
• What to eat: red meat, fish and shellfish give the iron that is absorbed best.
• Legumes and leafy greens have iron: eat them with citrus, kiwi or peppers, which help you absorb the iron in that meal.
• Keep tea and coffee away from meals: they slow absorption.
• Do not take iron supplements before your doctor has seen your ferritin: extra iron you do not need does not help.

When to worry

Talk to your doctor if:

• Your ferritin is low too.
• Your haemoglobin is low too.
• You are a man, or a woman after menopause, and your iron is low for no clear reason.
• You feel very tired or get short of breath on exertion.
• You see black stools or blood in your stools, are short of breath at rest, have chest pain or faint: call 112 or go to the emergency department.

Frequently asked questions

Low serum iron: do I need to take iron? Not on your own. First your ferritin is checked: if it is normal, you often do not need iron. If it is low, your doctor decides the dose and for how long.

Low iron but normal haemoglobin: what does it mean? That there is no anaemia for now. Check your ferritin: if it is low, your store is running down and this is the right time to act. If it is normal, it is often a day-to-day swing or inflammation.

Is low serum iron dangerous? Not by itself: it is a number that changes a lot. What matters is whether iron is really short, and ferritin shows that. A real deficiency needs treating, and its cause needs finding.

How often should it be checked again? If you feel well, with your usual checks: Rosa measures it once a year, together with ferritin and transferrin saturation. If your doctor starts iron treatment, follow their timing.

Sources

• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146
• Pasricha and colleagues, iron deficiency: causes, diagnosis and treatment. The Lancet, 2021. PMID 33285139
• Dale and colleagues, how serum iron, TIBC, transferrin saturation and ferritin vary through the day. American Journal of Clinical Pathology, 2002. PMID 12090432
• Nguyen and colleagues, the effect of time of day and fasting on serum iron. Clinical Biochemistry, 2017. PMID 28947322
• EASL clinical practice guidelines on haemochromatosis. Journal of Hepatology, 2022. PMID 35662478