Transferrins are iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids. Human transferrin is encoded by the TF gene.Transferrin glycoproteins bind iron tightly, but reversibly. Although iron bound to transferrin is less than 0.1% (4 mg) of total body iron, it forms the most vital iron pool with the highest rate of turnover (25 mg/24 h). The affinity of transferrin for Fe(III) is extremely high but decreases progressively with decreasing pH below neutrality. When not bound to iron, transferrin is known as “apotransferrin” (see also apoprotein). Transferrin has been shown to interact with insulin-like growth factor 2 and IGFBP3. Transcriptional regulation of transferrin is upregulated by retinoic acid.

An increased plasma transferrin level is often seen in patients suffering from iron deficiency anemia, during pregnancy, and with the use of oral contraceptives, reflecting an increase in transferrin protein expression. As follows, when plasma transferrin levels rise, there is a reciprocal decrease in percent transferrin iron saturation, and a corresponding increase in total iron binding capacity in iron deficient states. A decreased plasma transferrin can occur in iron overload diseases and protein malnutrition. An absence of transferrin results from a rare genetic disorder known as atransferrinemia.