Transferrin is a plasma glycoprotein produced by the liver that serves as the primary transport protein for iron in the blood. Each transferrin molecule can bind up to two iron atoms and delivers iron to cells expressing transferrin receptors — particularly developing red blood cells in the bone marrow, which require iron for haemoglobin synthesis.

Transferrin levels rise when iron stores are low — the body produces more transferrin to maximise iron capture from the blood. This makes transferrin useful in distinguishing iron deficiency anaemia (high transferrin) from anaemia of chronic disease (low or normal transferrin). Transferrin is also used to calculate transferrin saturation and TIBC, which together give a complete picture of iron status.