The soluble transferrin receptor (sTfR) is formed as a result of proteolytic cleavage of the membrane-bound molecule. In plasma, the soluble transferrin receptor is present together with transferrin in a complex that has a molecular weight of approximately 320 kD. The serum concentration of sTfR is directly proportional to the concentration of the receptor on cell membranes.
The function of the transferrin receptor is to take up the iron transported to the cell by transferrin and bring it into the cell. About 80% of transferrin receptors are localized on precursor cells of erythropoiesis. Erythrocytes are excluded. In any functional iron deficiency, that is, insufficient availability of iron for erythropoiesis, the number of receptors on the membrane is upregulated.
Since the transferrin receptors are continuously detached from the cell membrane and pass into the plasma as soluble transferrin receptors (sTfR), the concentration of sTfR in serum is an indicator of the iron supply to erythropoiesis. However, this serum concentration is not only determined by the receptor density, but also by the number of erythroblasts.
In the case of iron deficiency, the sTfR concentration increases even before there is a significant drop in the hemoglobin concentration. The sTfR concentration can therefore be used to describe the functional iron status, while ferritin provides an indication of the iron depot status.