The dominating mechanism for iron delivery to tissues is the internalisation of the transferrin receptor-diferric transferrin complex. Other ways for iron to enter cells exist but do not contribute significantly to the cellular iron homeostasis in humans. Once the complex has been endocytosed, iron is released to the cytosol for further transport inside the cell, for example to the mitochondria. The receptor-apotransferrin complex recycles to the cell surface for a renewed round of iron uptake. A fraction of the transferrin receptors are cleaved inside the endocytotic vesicle and finally shed into the blood as truncated transferrin receptor monomers complexed with apotransferrin. The serum concentrations of these so called serum transferrin receptors (sTfR) correlate with erythropoietic activity and tissue iron demands. This review will focus on the clinical value of sTfR measurements.
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