Ferritin

Your body's iron store. It falls before hemoglobin does, which makes it the earliest routine sign of iron deficiency.

Low ferritin with a still-normal hemoglobin explains fatigue, hair loss and poor exercise recovery on its own.

Ferritin also rises with any inflammation, so a normal value taken during an infection can hide empty stores. Many clinicians treat anything under 30 as low whatever the printed range says.

Worth discussing a low or falling value even while hemoglobin is still normal.