Transferrin saturation (TSAT) expresses the percentage of transferrin binding sites that are currently occupied by iron. It is calculated from serum iron divided by TIBC, multiplied by 100. Normal TSAT is typically 20–50%.

Low TSAT (below 20%) indicates iron deficiency — little iron is available to bind to transferrin. High TSAT (above 50%) indicates iron overload — transferrin is saturated and non-transferrin-bound iron may accumulate in organs, causing the tissue damage seen in haemochromatosis. TSAT is an essential component of a complete iron panel, providing the most direct measure of iron availability for red blood cell production.