Transferrin Saturation

Transferrin saturation (TSAT) tells you what percentage of your iron transport protein is actually carrying iron. It is calculated by dividing your serum iron by your total iron binding capacity (TIBC) and multiplying by 100. Think of it as how full the iron delivery lorries are, a low percentage means most lorries are running empty, while a high percentage means they are packed. TSAT is widely regarded as one of the most reliable single markers for assessing iron status because it combines supply (serum iron) and demand (TIBC) into one neat figure.

A low TSAT is one of the earliest and most specific signs of iron deficiency, often dropping before haemoglobin or ferritin show clear changes. A high TSAT can be an early signal of iron overload. Normal TSAT typically sits between 20% and 45%. Below 20% suggests your tissues may not be receiving enough iron, which can lead to fatigue, poor concentration, and reduced exercise tolerance. Above 45% may indicate the body is accumulating more iron than it can safely use, which warrants further investigation.

Because TSAT depends on serum iron, which fluctuates during the day and with meals, it is best measured in the morning after fasting. A single abnormal reading should usually be confirmed with a repeat test. TSAT is particularly valuable when ferritin is hard to interpret, for example, during illness or inflammation, when ferritin may appear normal despite genuine iron deficiency. In these situations, a low TSAT helps reveal the true picture. If your transferrin saturation falls outside the expected range, your GP can advise on next steps, whether that involves dietary changes, supplementation, or additional testing.