Phases of advancing ID. The gradual diminution of the different iron compartments and the concomitant changes seen in the laboratory analytes are presented schematically in relation to the separate stages of advancing ID. Typical laboratory findings are displayed separately for each stage, and the normal values of sTfR, ferritin, and sTfR/log ferritin (TfR-F Index) are presented as 95% reference intervals obtained from our study population of apparently healthy adults after 3 months of oral iron supplementation. Normal Hb concentrations are the reference values currently applied at the Turku University Central Hospital (TUCH). The criteria used to define the different stages of ID appear boxed. The prevalences of the different subgroups with various degrees of ID in our study population before the supplementation are presented as determined by these criteria.