Ferritin is a protein that stores iron. The body increases serum ferritin production when excess iron is absorbed, so serum ferritin levels are a good indication of the body's iron stores.
Note that serum ferritin values may be artificially increased by underlying cancer or inflammatory/infectious conditions.
Iron is present in greatest concentration in meat and dark green vegetables. The average daily American diet contains about 10mg iron, of which only about 1mg is absorbed. What goes in must come out, and in the adult male, the 1 mg/day iron loss occurs almost exclusively in the stool. For reproductive-aged females, an additional route is the menstrual flux, which accounts for a wildly variable incremental loss. While the average monthly menstrual blood loss is 40mL (equivalent to 16mg iron), some women who consider themselves healthy may lose up to 495mL blood (about 200mg iron) per menstrual period, or an average of about 7mg iron per day (200mg iron ÷ 28 days/cycle). It is not surprising that iron deficiency anemia is relatively common in women of this age group.
The U.S. Recommended Daily Allowance for adults is 10mg for males and 18mg for menstruating females.
Patients with elevated transferrin saturation test values should proceed with serum ferritin testing and additional workup as warranted.
Consistently excessive blood loss will result in a negative iron balance which will result in lowered serum ferritin levels. Iron deficiency is a cause as well as consequence of menorrhagia. Body stores of iron are best reflected by serum ferritin. Ferritin levels will drop before changes in the blood count begin. Anemia may not yet be present when serum ferritin levels are getting low.