An iron panel prints four to six rows, and they are not four attempts at measuring iron. They measure different things: what is circulating, what is stored, and how much carrying capacity exists.
Serum iron: what is in circulation now
MedlinePlus: serum iron "measures the amount of iron in the blood".
This is the iron moving through your bloodstream at the moment of the draw — a snapshot, and the row most sensitive to when the sample was taken.
Ferritin: what is in storage
MedlinePlus describes ferritin as "a protein that binds to iron and stores it in your body", and the test as one that "can show how much iron you have stored in your body". The iron-tests page puts it the same way: ferritin "measures how much iron is stored in the body".
So ferritin is not circulating iron measured a second way. It is a different pool.
Transferrin and TIBC: the carrying capacity
MedlinePlus: transferrin is "a protein that moves iron throughout the body", and a transferrin test "measures transferrin".
TIBC — total iron-binding capacity — "measures how well iron attaches to transferrin and other proteins in the blood". It describes capacity rather than contents: how much iron the blood could carry, not how much it is carrying.
UIBC — unsaturated iron-binding capacity — is the unused portion of that capacity. TIBC, UIBC and serum iron are related arithmetically, which is why a report printing all three is not printing three independent measurements.
Transferrin saturation: a calculated percentage
This row is serum iron expressed as a share of the total capacity. It is division, not measurement — which puts it in the same category as the other calculated rows on a report.
Because it is a ratio, it moves when either input moves. A change in transferrin saturation does not by itself say whether circulating iron or carrying capacity changed; the two rows above it do.
Why the panel has this shape
Because a single iron measurement is ambiguous. Circulating iron, stored iron and carrying capacity can move independently, and the panel exists to separate them.
MedlinePlus does not describe UIBC or transferrin saturation on the pages cited here, so this guide describes what those rows are arithmetically — how they relate to the measured values printed beside them — rather than what any particular value indicates. What a pattern across these rows means for a person is a clinical question and belongs with a doctor.