Where your iron goes: Haemoglobin, ferritin, myoglobin and transferrin explained

In the average adult, total body iron is typically 3-4 g, with most of it found in red blood cells. It might not sound like much, but that iron is vital to an array of bodily functions.

Think of iron as stock in a busy business. Haemoglobin is the delivery team, moving oxygen around the body in red blood cells. Ferritin and haemosiderin are the stockroom, holding iron in reserve until it is needed. Myoglobin keeps the workshop running by helping muscles use oxygen. Transferrin is the transport service, carrying iron through the blood to the places that need it.

When that system starts to run short, you may notice the effects before you understand the cause. Energy drops. Stairs feel harder. Concentration slips. Behind those everyday signs, iron is being carried, stored, released and reused across different proteins.

What are the main forms of iron in the body?

The main forms are haemoglobin iron, storage iron, myoglobin iron and transferrin-bound iron. They do different jobs, so one “iron level” rarely tells the full story.

A useful map of where iron sits in the body looks like this:

• Haemoglobin: about 65% of body iron, mainly for oxygen transport in red blood cells.

• Ferritin and haemosiderin: up to about 30%, mainly stored in the liver, spleen, bone marrow and muscle.

• Myoglobin: about 3.5%, mainly in muscle.

• Transferrin-bound iron: about 0.1%, a small travelling pool in the blood.

• Other iron: small amounts in enzymes and proteins used in energy production and cell function.

Think of that business. Haemoglobin serves customers all day. Ferritin and haemosiderin sit in the back room. Myoglobin keeps the operations running during busy spells. Transferrin is the delivery van moving stock to the right place.

What is haemoglobin and why does it use so much iron?

Haemoglobin is the oxygen-carrying protein in red blood cells. It uses iron because each haem group can bind oxygen, then release it to tissues.

Haemoglobin is made from four globin protein chains, each attached to a haem unit. Each haem unit contains one iron atom. This is why low iron can eventually affect red blood cell production and oxygen delivery. Around 65% of total body iron is found in haemoglobin, making it the largest iron pool.

When doctors check for anaemia, they often start with a full blood count. This can show haemoglobin concentration, red blood cell size and related markers. A GP will usually use a full blood count to check whether the number of red blood cells is normal when iron-deficiency anaemia is suspected.

Low haemoglobin can come with symptoms such as:

• Tiredness and low energy

• Shortness of breath

• Noticeable heartbeats

• Paler skin than usual

• Headaches

A normal haemoglobin result does not always mean your iron stores are fine. Storage iron can fall first, before anaemia appears.

What is ferritin, and how is it different from haemosiderin?

Ferritin is the main soluble iron-storage protein. Haemosiderin is a less soluble storage form, used more when iron stores are greater or when storage is longer-term.

Ferritin acts like neat shelving. It stores iron inside a hollow protein shell made from 24 subunits, and one ferritin molecule can hold around 4,000 to 4,500 iron atoms. Haemosiderin is more like stock packed into harder-to-reach crates. It is less soluble and tends to sit in storage cells, including cells in the liver and bone marrow.

Storage iron can account for up to about 30% of total body iron. Around two-thirds of storage iron is ferritin, with the rest in haemosiderin. Serum ferritin, the ferritin measured in blood, usually tracks body iron stores in healthy people without inflammation.

That makes ferritin useful when symptoms are vague. Fatigue may come from low iron stores, low vitamin D, thyroid issues, poor sleep, stress, infection, or several other causes. Where it fits, Rezure’s Iron Deficiency (Ferritin) Rapid Home Test Kit checks ferritin from a small blood sample and is positioned for people who want a quick view of their iron stores at home.

If tiredness is the broader concern, low vitamin D and thyroid imbalance can also fall within the same symptom area. Rezure’s Vitamin D Rapid Home Test Kit checks vitamin D status with a finger-prick sample, while the Underactive Thyroid (TSH) Rapid Home Test Kit measures TSH, a marker often used when an underactive thyroid is suspected.

Why test ferritin if haemoglobin is what carries oxygen?

Ferritin can fall before haemoglobin drops. That means ferritin testing can show low iron stores before clear iron-deficiency anaemia develops.

This matters because the body tries to maintain haemoglobin production for as long as possible. When intake, absorption or blood loss tips the balance, stored iron may be used first. A person may have low ferritin with a normal full blood count, especially early on.

Ferritin below 30 micrograms/L indicates iron depletion, so ferritin is a first-line test in the investigation of anaemia, and low ferritin often appears before anaemia.

Ferritin also has a catch. It can rise during inflammation, infection, liver disease, heart failure and some other conditions. A “normal” or high ferritin result may need professional interpretation if symptoms continue or inflammation is suspected.

A ferritin check may be useful when you have symptoms or risk factors such as:

• Ongoing tiredness, dizziness or breathlessness

• Heavy periods

• Pregnancy or recent pregnancy

• Frequent blood donation

• A vegan or vegetarian diet

• Possible absorption issues, such as coeliac disease or gut problems

What is myoglobin, and does it show iron deficiency?

Myoglobin is an oxygen-binding protein in muscle. It contains one haem unit and one globin chain, and accounts for about 3.5% of body iron.

Its job is local. Haemoglobin carries oxygen around the body. Myoglobin helps muscles hold and use oxygen where it is needed. This matters for movement, stamina and muscle function, although myoglobin is not usually tested to assess iron stores.

In routine iron assessment, doctors usually look at haemoglobin, ferritin, serum iron, transferrin, TIBC and transferrin saturation instead. Myoglobin blood tests are used in medicine, but they are primarily used to assess muscle injury rather than for everyday iron deficiency tests.

What is transferrin, and what does transferrin saturation mean?

Transferrin is the main iron-transport protein in the blood. It carries iron to the bone marrow, liver, spleen and other tissues.

Transferrin is made mainly by the liver. It binds ferric iron tightly, helping keep free iron low in the blood. Each transferrin molecule can bind two ferric iron atoms. Although transferrin-bound iron makes up only about 0.1% of total body iron, around 20 to 30 mg moves through this transport pool each day.

Transferrin saturation shows how much of transferrin’s iron-carrying capacity is being used. Low transferrin saturation can support a picture of iron deficiency, but it should be interpreted alongside other markers.

TIBC measures the blood’s ability to bind to iron and transport it throughout the body. In iron-deficiency anaemia, iron is low, while TIBC is high. In iron overload, iron is high while TIBC is low or normal.

Professional iron studies may include:

• Full blood count, to assess haemoglobin and red blood cell patterns.

• Serum ferritin, to estimate stored iron.

• Serum iron, usually read with other tests, because it varies across the day.

• TIBC (transferrin) to assess iron transport capacity.

• Transferrin saturation, indicating how much of the transport capacity is filled.

• CRP or other inflammation markers, when ferritin may be harder to interpret.

Can high ferritin mean something different from low iron?

Yes. Low ferritin usually points towards low iron stores, but high ferritin does not simply mean “high iron”.

Ferritin can rise when the body has inflammation, liver disease, kidney disease, heart failure, excess alcohol intake, malignancy or hyperthyroidism. It can also rise with true iron overload. This is why clinicians may check transferrin saturation, liver markers, inflammation markers and medical history before drawing conclusions.

This is also where separate health checks can get mixed up in people’s minds. Ferritin is not a cholesterol test, and cholesterol does not assess iron. If your concern is heart and vascular risk, a Total Cholesterol Rapid Home Blood Test Kit sits in a different category. It checks total cholesterol, while ferritin testing looks at iron stores.

How should you think about iron testing as a whole?

Think of iron testing as checking different parts of the same business. Haemoglobin shows whether oxygen delivery is keeping up. Ferritin indicates how much iron is stored. Transferrin shows how iron is being moved around the body. Myoglobin is the workshop floor: it helps muscles use oxygen for physical work, even though it is not usually used to check for low iron stores.

The practical point is simple: low iron is not only about anaemia. Your stored iron can fall before your haemoglobin drops, which is why ferritin testing can be useful. Professional lab tests can then add more context, especially when symptoms, medical history or inflammation make the picture less clear.

If you feel persistently tired, breathless, dizzy or unusually weak, or you notice palpitations, speak to a healthcare professional. This matters more if your symptoms are new, worsening or hard to explain.