Why does serum contain ferritin?

Abstract

In a recent article, Koperdanova and Cullis 1 rehearsed the various interpretations of raised serum ferritin levels. In fact the most interesting question 2 is why does ferritin appear in serum at all? All modern network models of iron metabolism (e.g. 3-7) have iron being passed from the gut to peripheral cells via blood (serum) bound to transferrin, a well-established iron-transporting molecule that is present in serum at ca 0.6-3.3 g.L-1 8. By contrast, ferritin is an intracellular iron storage compound 3 5 7; its normal range for serum is at levels 10,000-100,000 times lower than that for transferrin, being from very small levels to up to 300 μg.L-1 in men and slightly lower in women 2.