Serum iron measures the concentration of iron circulating in the blood bound to transferrin, the iron transport protein. It represents a small and highly variable fraction of total body iron. Unlike ferritin (which reflects stable iron stores), serum iron fluctuates significantly throughout the day (diurnal variation), after meals, and in response to inflammation. It is most useful when interpreted alongside transferrin saturation and ferritin as part of a complete iron studies panel.

Iron is essential for haemoglobin synthesis, cellular energy production, DNA synthesis, and immune function. Deficiency impairs oxygen delivery and energy metabolism; excess causes oxidative damage to organs. Serum iron alone is insufficient to diagnose iron deficiency or overload and should always be interpreted with the full iron studies profile.