Transferrin

The protein that carries iron around the blood. The liver makes more of it when iron is short.

It moves in the opposite direction to your iron stores, which makes it a useful cross-check on ferritin.

High transferrin with low ferritin is the classic pattern of true iron deficiency. It falls with inflammation and with poor nutrition, so a normal value during an illness is not reassuring. Some reports print total iron-binding capacity instead, which measures the same thing.

Worth reading with ferritin and saturation as a set.