Transferrin saturation (TSAT) shows what fraction of the transport protein transferrin is actually loaded with iron. It is calculated simply: serum iron divided by TIBC, in percent.
It is one of the best indicators of iron availability "here and now". Together with ferritin (the store), saturation (the transport) gives the full picture: both the warehouse and the logistics.
Reference ranges
The reference depends on the lab; approximate values:
| Normal | 20 – 45% |
| Iron deficiency | < 20% |
| Iron overload | > 45–50% (suspect hemochromatosis) |
| Assessed with ferritin | store + transport together |
What an elevated level means
High transferrin saturation means an excess of available iron. The main cause you must not miss is hereditary hemochromatosis (iron overload): persistently high TSAT (usually >45–50%) together with high ferritin is a reason for further evaluation.
Other causes: repeated blood transfusions, excessive iron supplementation, some liver diseases.
What a low level means
Low transferrin saturation is an early and sensitive sign of iron deficiency: there is plenty of transport protein but nothing to load into it. It is usually accompanied by low ferritin and high transferrin and TIBC.
Low TSAT also occurs with inflammation (anemia of chronic disease), where iron is present but "locked away" and unavailable to the tissues.
How the test is done
Saturation is not measured directly — it is calculated from serum iron and TIBC. So it is taken as part of the full iron profile (iron, ferritin, transferrin/TIBC) and interpreted together.
The result is affected by the time of day and recent iron intake — test in the morning fasting, and stop iron supplements as advised by the lab.
What affects the value
- Time of day — iron has a daily rhythm (higher in the morning).
- Recent iron intake — temporarily inflates it.
- Iron deficiency — lowers saturation.
- Inflammation — lowers it (iron is "locked away").
- Hemochromatosis and transfusions — raise it.
- Pregnancy, estrogen — act via transferrin.
Assess iron deficiency
Transferrin saturation is a key marker of iron status; assess your deficiency risk:
Frequently asked questions
How is transferrin saturation better than ferritin?
Ferritin is the iron store, but it rises with inflammation and can "mask" deficiency. Transferrin saturation reflects iron availability to the tissues right now and, together with ferritin, helps avoid missing deficiency even with inflammation.
Is saturation above 45% hemochromatosis?
Not a diagnosis, but a reason to be evaluated. Persistently high saturation (>45–50%), especially with high ferritin, is a typical sign of iron overload and hereditary hemochromatosis. It is confirmed with a genetic test and repeat labs.
Saturation is low but ferritin is normal — why?
This happens with inflammation: ferritin rises as an acute-phase protein and looks "normal", although available iron is low. A low saturation here helps recognize functional iron deficiency.
Related terms
Sources
- Iron-Binding Capacity — StatPearls (NCBI)
- MedlinePlus (NIH) — Iron Tests
- MedlinePlus (NIH) — Hemochromatosis