IronSerum iron
The iron travelling in your blood right now, which is not the same as your iron stores.
Serum iron measures the iron currently in transit in the blood, nearly all of it bound to the transport protein transferrin. It is a snapshot of circulating iron, not of how much the body has in reserve.
It matters because that distinction is routinely misread. Serum iron swings substantially through the day and rises after an iron-containing meal or supplement, whereas ferritin reflects stored iron and is the more stable measure of iron status.
In practice serum iron is interpreted with transferrin, total iron binding capacity, transferrin saturation and ferritin together; on its own it can be misleading in either direction.