Serum iron: what it is, what it tells you and why we test it

Serum iron is the iron circulating in your blood right now. It does not show how much iron you have put aside: that is ferritin. It changes a lot from day to day, which is why it is read together with other tests.

Why we test it

Iron carries oxygen to your muscles and brain. In the blood it travels bound to a carrier protein, transferrin: serum iron measures this iron.

On its own it says little, because it changes a lot. But it is used to calculate transferrin saturation, which shows how much iron is ready to use.

Read with ferritin and saturation, it helps show whether iron is short or there is too much.

What it tells you, and what it does not

If it is low, iron may be short. But it also falls with inflammation or an infection, even when your store is full.

If it is high, an iron supplement taken in the days before is often behind it. More rarely, the body builds up too much iron.

On its own it is not enough for either: ferritin shows your store.

That is why it is read together with other tests:

• Ferritin: Your iron store
• Transferrin saturation: How much iron is ready to use
• Haemoglobin: Whether there is already anaemia
• TIBC: How many seats there are for iron in total

Normal serum iron levels

The lab decides what is normal. It is usually 33 to 193 µg/dL. Your own report may print different limits, sometimes separate ones for women and men.

It is a wide range because the number changes a lot: from one person to another, from day to day and even within the same day.

That is why a number just outside the limits says little on its own. What counts is how it reads with ferritin and transferrin saturation.

In the Rosa report you also see your optimal range, alongside ferritin and transferrin saturation.

What your result means

Find the row for your result, in µg/dL. On its own, serum iron cannot tell whether iron is short or too high: check ferritin and transferrin saturation too.

• 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.
• Above your report’s limit, Usually above 193: High. An iron supplement taken in the days before the test is often behind it. Check your transferrin saturation and ferritin. If they are high too, talk to your doctor.

If your serum iron is high or low

• Low serum iron: Iron is often short, but sometimes inflammation alone is behind it. Ferritin tells you which.
• High serum iron: An iron supplement taken in the days before is often behind it. The causes are below.

High serum iron: the causes

It usually does not mean too much iron. The first two causes are the most common.

• An iron supplement: Even a single tablet raises it for hours. If you took one in the two days before the test, it is the most likely cause.
• A day-to-day swing: It changes a lot from one day to the next. If ferritin and saturation are normal, a number just above the limit is usually nothing to worry about.
• Too much iron in the body: Rarer. For example with haemochromatosis, an inherited condition in which the body holds on to too much iron. Transferrin saturation and ferritin show it.
• Liver disease: A liver that is unwell can release iron into the blood.

What can skew the result

A few things change the number for a short while. It helps to know them before you read it.

• Iron supplements: They raise it for hours. If you take them, ask your doctor whether to pause them for a day or two before the test.
• A meal: After eating, the number changes for a few hours: best to have the test in the morning, fasting.
• Inflammation or infection: They push it down even when your store is full. A flu can be enough.
• Time and day: It changes within the same day too, but not always in the same direction. To compare two tests, it helps to do them the same way.

When to worry

Talk to your doctor if:

• It is low and your ferritin or haemoglobin is low too.
• It is high, you have not taken iron in the days before, and your transferrin saturation is high too.
• It stays out of range from one test to the next and you do not know why.
• You see black stools or blood in your stools: call 112 or go to the emergency department.

Frequently asked questions

Are serum iron and iron the same thing? Yes. On an Italian report you may see it as sideremia, ferro or ferro sierico: it is the same test, the iron circulating in your blood.

Is serum iron enough to tell whether I am short of iron? No. It changes a lot from day to day and falls with inflammation. Ferritin, read with transferrin saturation, shows whether iron is short.

Do I need to fast for a serum iron test? Yes, it is better. A meal or an iron supplement changes it for a few hours. You can drink water.

Does a full blood count include iron? No. The blood count counts and measures your blood cells. Serum iron and ferritin have to be asked for separately: the Rosa panel has both.

How often should it be checked? For people who feel well there is no fixed rule: 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