A measure of iron bound to transferrin and serum proteins. It represents 1/3 of the total iron binding capacity of transferrin. 70% of iron is bound to haemoglobin. On its own, it is a relatively poor marker of iron status. Levels may indicate haemochromatosis, haemolytic anaemia, liver damage, hypochlorhydria, Vit B6 deficiency, poor iron intake or absorption, blood loss, chronic disease, pathogen infection, progesterone birth control pills.