Biomarker 101: Iron studies

Iron studies return four or five numbers, and none of them means much alone. Read as a set, they show not just whether iron is low but how far along that process has gone.

Why iron matters

Iron is used to make haemoglobin, the protein in red blood cells that carries oxygen from your lungs to your tissues. Your body cannot manufacture it, so it all comes from diet.

It also has no route of disposal. There is no mechanism for excreting excess iron, so the body regulates its supply entirely by controlling how much is absorbed from food. That asymmetry explains both ends of the problem: too little causes iron deficiency anaemia, and too much accumulates in organs.

How iron moves through the body

Iron is absorbed in the small intestine, then binds to a transport protein called transferrin, which carries it around the body. Storage happens in ferritin, mostly in the liver, which releases iron when blood levels fall.

Released iron binds to transferrin again and travels to the bone marrow, where developing red blood cells take it up. Since red cells live around 120 days, the demand is constant — though old cells are recycled and their iron reclaimed.

What's in the panel

Ferritin reflects stored iron. It is usually the most informative single marker, because stores deplete first.

Transferrin measures transport capacity. Production rises when iron is scarce and falls when it is plentiful — so it moves in the opposite direction to your stores. Some laboratories report total iron binding capacity instead, an older test measuring the same thing.

Serum iron is the amount circulating right now. It varies considerably through the day and is rarely useful alone; its main role is in calculating saturation.

Transferrin saturation combines the previous two to show what percentage of transport capacity is actually carrying iron. It typically sits between 15 and 45 per cent — low in deficiency, high in overload.

How the pattern develops

Iron deficiency is gradual, and the panel shows which stage you are at.

Stores go first: ferritin falls while serum iron and transferrin remain normal, and there is no anaemia. As it progresses, serum iron drops, transferrin rises to compensate, and saturation falls. Only later does haemoglobin production suffer, producing red cells that are smaller and paler than usual.

This is why ferritin can be low while a full blood count looks entirely normal — and why testing stores rather than waiting for anaemia catches it considerably earlier.

The opposite pattern — high ferritin and serum iron with low transferrin and high saturation — suggests iron overload, which may relate to an inherited condition called haemochromatosis.

What iron studies won't tell you

• Ferritin rises with inflammation. Recent illness can lift it independently of your actual stores, which sometimes masks deficiency.
• They don't identify why. Low iron has many causes, and finding the reason matters more than the number.
• Don't supplement on your own. Iron supplements are not harmless, and taking them without knowing your status can cause problems.
• Timing affects the result. Iron supplements and recent food both shift serum iron, so follow the collection instructions.

The bottom line

Iron studies work as a set because each marker describes a different stage of the same process — storage, transport and availability. Read together they show not only whether iron is low, but how long it has been heading that way.