Iron, Total and Total Iron Binding Capacity (TIBC)

Serum iron measures iron bound to transferrin in the circulation, which represents only a small transit pool — roughly 3 to 4 mg — against total body iron of approximately 3 to 4 grams, most of which resides in haemoglobin and in ferritin-bound tissue stores. Circulating iron is subject to a pronounced diurnal rhythm, with morning values typically highest and evening values substantially lower, and within-day variation commonly exceeds 30%. It rises within hours of oral iron ingestion or an iron-rich meal, and falls acutely in inflammation through hepcidin-mediated sequestration.

Serum iron in isolation is of very limited clinical value and should not drive management decisions on its own, because its marked diurnal variation, sensitivity to recent intake, and acute suppression by inflammation mean a single value poorly reflects iron status. Its utility lies in being combined with total iron binding capacity to calculate transferrin saturation, and in the pattern the three values form together. It is also relevant in suspected acute iron overdose, where the absolute concentration is directly meaningful.

Total iron binding capacity measures the serum's total capacity to bind iron and functions as an indirect measure of transferrin, the principal iron transport protein synthesised by the liver. Transferrin production is upregulated in iron deficiency as a compensatory response, raising TIBC, and is reduced in inflammation, malnutrition, hepatic disease, and protein-losing states, since transferrin is a negative acute-phase reactant. TIBC is more stable across the day than serum iron and is not materially affected by recent iron intake.

TIBC provides the denominator for transferrin saturation and contributes essential discriminatory information to the iron studies pattern. A raised TIBC alongside low serum iron is characteristic of true iron deficiency, whereas a low or normal TIBC with low serum iron points towards anaemia of chronic disease or inflammation. Because transferrin is a negative acute-phase reactant, TIBC falls in inflammation, hepatic disease, malnutrition, and nephrotic syndrome, and rises with oestrogen therapy and in pregnancy.

Transferrin saturation is calculated rather than measured directly, as the ratio of serum iron to total iron binding capacity expressed as a percentage. It represents the proportion of available iron binding sites that are occupied, and therefore reflects the balance between circulating iron and transport capacity better than either value alone. Because it inherits the diurnal variation and dietary sensitivity of its serum iron numerator, it is best measured on a fasting morning specimen.

Transferrin saturation is the most clinically useful single output of this panel. A saturation above 45% is the established screening trigger for hereditary haemochromatosis and iron overload, and is the panel's strongest validated application. A saturation below 20% supports iron deficiency and is used in the assessment of anaemia and in monitoring iron therapy, including in chronic kidney disease where ferritin is unreliable because of chronic inflammation. It remains a calculated value inheriting the variability of serum iron, so a single result should be confirmed before acting on it.