Total iron binding capacity, transferrin, saturation, and ferritin

Total iron binding capacity (TIBC) measures how much iron your blood's transport protein could carry if every binding site were full. It's an indirect read on transferrin, usually reported in mcg/dL. Labs pair it with serum iron to calculate transferrin saturation.

What total iron binding capacity actually measures

Iron doesn't float loose in plasma. Free iron is reactive, so the body keeps it chaperoned in transit by transferrin — a liver-made glycoprotein that carries up to two ferric (Fe³⁺) ions per molecule. Total iron binding capacity estimates how much iron all the transferrin in a sample could hold at once.

Older assays measured it head-on: saturate the serum with iron until every site is occupied, strip the excess, weigh what stayed bound. Many labs now quantify transferrin by immunoassay and convert, which is why the two results track each other so closely. A TIBC blood test rarely arrives alone — it's bundled as an "iron panel" or "iron studies" with serum iron and usually ferritin.

Two relationships explain most of what the panel prints, since labs often measure one value and calculate the rest:

• UIBC (unsaturated iron binding capacity) = TIBC − serum iron
• Transferrin saturation (%) = (serum iron ÷ TIBC) × 100

Adult TIBC reference intervals commonly land near 250–450 mcg/dL, roughly 45–80 µmol/L, though every lab publishes its own.

Serum iron: who's on board right now

Serum iron counts the iron currently riding on transferrin — not what's stored, not what you ate. A typical printed serum iron normal range sits around 60–170 mcg/dL (roughly 10–30 µmol/L), with some labs splitting the interval by sex.

This is the least stable number on the panel: it follows a diurnal rhythm, peaking in the morning and drifting down through the day, and a supplement or iron-heavy meal can lift it for hours — which is why draws are usually early and fasting, and why one value in isolation carries little weight.

Transferrin saturation: how full the seats are

If transferrin is the bus and TIBC counts the seats, saturation is the percentage of seats occupied. It carries more information than either input alone because it normalizes supply against capacity.

Commonly reported adult intervals run about 20–50%, though plenty of labs print 15–45%, and men average slightly higher than women. The textbook patterns pair low saturation with a high TIBC — transferrin production ramps up when stores run low — and high saturation with a TIBC that's normal-to-low, since the liver has less reason to keep churning out transport protein once iron is already abundant. Many US screening algorithms use a saturation at or above 45% as the trigger for further iron-overload evaluation. What any individual result means belongs to the clinician who ordered the panel; the numbers here describe what labs print, nothing more.

Ferritin: the warehouse, not the traffic

Ferritin is the storage protein. One shell holds thousands of iron atoms — commonly cited as up to roughly 4,500 — and most of the body's ferritin sits inside cells of the liver, spleen, and bone marrow. The small fraction that leaks into serum is what gets measured, and it tracks body iron stores closely enough to be the first-line storage marker. A frequently used approximation puts 1 ng/mL of serum ferritin near 8–10 mg of stored iron.

Printed ranges vary widely — often something like 30–400 ng/mL for men and 15–200 ng/mL for women. The WHO uses a serum ferritin below 15 µg/L to define depleted stores in adults.

The catch: ferritin is also an acute-phase reactant. Infection, chronic inflammation, liver disease, and malignancy raise it independently of how much iron is banked, which is why CRP is often drawn beside it and why WHO applies a higher cutoff when inflammation is present. More in our guide to how ferritin reflects iron stores.

The iron panel at a glance

Marker	What it measures	Usual units	Commonly printed adult range
Serum iron	Iron bound to transferrin right now	mcg/dL (µmol/L)	~60–170 (10–30)
Transferrin	Concentration of the transport protein	mg/dL (g/L)	~200–360 (2.0–3.6)
TIBC	Total iron that transferrin could bind	mcg/dL (µmol/L)	~250–450 (45–80)
UIBC	Binding sites still empty	mcg/dL	~150–375
Transferrin saturation	Serum iron ÷ TIBC, as a percentage	%	~20–50 (some labs 15–45)
Ferritin	Proxy for stored iron	ng/mL (µg/L)	~30–400 men, ~15–200 women
Hepcidin	Hormone gating iron release into blood	ng/mL	no standardized range

Intervals differ by lab, assay, age, and sex; use the one printed on your own report.

Hepcidin: the gate on the gut wall

Hepcidin is a 25-amino-acid peptide hormone made by the liver, and it's the reason dietary iron and blood markers behave like loosely coupled systems. Its target is ferroportin, the only known protein that moves iron out of a cell and into the bloodstream. When the hormone binds ferroportin, the channel is occluded and degraded — the export door closes.

Three cell types matter here: duodenal enterocytes shipping newly absorbed dietary iron, macrophages recycling iron from worn-out red cells, and hepatocytes releasing what's in storage. All of them export through ferroportin, so one hormone throttles all three at once. Iron taken up by an enterocyte while that door is shut never reaches the blood at all; it leaves with the cell when the intestinal lining turns over a few days later.

The signals are directional. Iron loading and inflammation, largely via IL-6, push hepcidin up. Iron deficiency, hypoxia, and active red-cell production push it down — erythroblasts release erythroferrone, which suppresses the hormone when the marrow needs raw material. Assays exist but aren't standardized across methods, so this stays a research measurement rather than a routine panel line.

The practical consequence is that absorption is a variable, not a constant. Estimates of how much dietary iron crosses the gut wall commonly span roughly 5–35%, depending on iron status and what else is on the plate. Heme iron from meat and seafood absorbs more efficiently and is less sensitive to meal inhibitors than non-heme iron — the difference between heme and non-heme iron explains much of why two people eating identical milligram totals absorb different amounts.

Why intake and blood markers rarely move together

Each marker runs on its own clock. Serum iron shifts within hours. Transferrin and TIBC respond over days to weeks. Ferritin moves over weeks to months. Red cells live about 120 days, so hemoglobin reflects a supply situation from months back. A panel is one morning inside all of that.

Intake sits upstream of all of it, in a different unit entirely. The NIH lists an RDA of 8 mg/day for men 19–50 and 18 mg/day for women 19–50, dropping to 8 mg/day for women after 50 and rising to 27 mg/day in pregnancy; vegetarians and vegans are advised to aim for about 1.8 times the RDA, since non-heme iron absorbs less efficiently. The tolerable upper intake level for adults is 45 mg/day. Our breakdown of daily iron intake in milligrams covers how those figures shift by age.

Because the RDA is a daily figure while blood markers integrate weeks of behavior, a food log is what makes a panel legible over time. Iron Tracker is an iron intake tracking app for iPhone; logging meals gives the next draw a companion timeline instead of one isolated snapshot.

FAQ

What is TIBC on a blood test?

TIBC is the total amount of iron the transferrin in your sample could bind if every site were occupied. It's reported in mcg/dL, with adult reference intervals typically near 250–450, and its main job is feeding the transferrin saturation calculation.

What is transferrin?

Transferrin is the plasma protein that transports iron. It's made in the liver, each molecule binds up to two ferric ions, and it delivers them to cells — mostly to bone marrow building red cells. Serum transferrin is often reported around 200–360 mg/dL; TIBC expresses the same information as binding capacity.

What is a normal transferrin saturation range?

Most labs print a transferrin saturation normal range of roughly 20–50%, and some use 15–45%. The value is serum iron divided by TIBC, times 100. Because both inputs move with time of day and assay method, saturation is read alongside ferritin rather than on its own.

Is hepcidin measured on a standard iron panel?

No. Those assays are used in research and specialty settings, and results aren't standardized across methods. A routine panel reports serum iron, TIBC, transferrin saturation, and ferritin instead.