Serum iron is the circulation of today; ferritin is the reserve. Stores drain first - ferritin falls while iron still reads normal - and inflammation can falsely lift ferritin.
What to do
Common questions
What is the difference between iron and ferritin?
Iron and ferritin measure different things, and confusing them causes missed diagnoses. Serum iron is the iron circulating in your blood right now - a snapshot that swings with meals, time of day, and inflammation. Ferritin is the storage protein - the reserve tank - and it reflects your total body iron stores, stable over days. MedlinePlus frames the ferritin test as the best single measure of iron stores, which is why it catches deficiency that a normal serum iron hides. The classic trap: early iron deficiency drains the reserve first - ferritin falls while serum iron and hemoglobin still read normal. People get told their iron is fine while their stores are empty and their fatigue has a name. The reverse trap: ferritin is also an inflammation marker, so infections and chronic inflammatory conditions can push it up, masking low stores - which is why providers sometimes pair it with CRP or transferrin saturation. The practical reading: ferritin below about 15 to 30 ng/mL indicates depleted stores whatever the hemoglobin says; fatigue, hair shedding, and restless legs at low-normal ferritin are worth a treatment trial; and repletion takes months - recheck at two to three months, aiming to rebuild the reserve rather than nudge the number over the line. Pymander is a free AI doctor, 24/7 by text. Share both numbers if you have them, and it can help you read the pair.