Serum iron test: what it measures, normal iron levels, why it is read with ferritin and TIBC, and what low or high iron means

Almost all of the body's iron is inside cells — in hemoglobin, in muscle myoglobin, and stored in ferritin, mostly in the liver, spleen and bone marrow. Only a tiny fraction, about three or four milligrams, is in transit in the blood at any moment, bound to the transport protein transferrin. The serum iron test measures that transit pool, in micrograms per deciliter (mcg/dL, sometimes written µg/dL).

Because the transit pool is small and turns over quickly, it is sensitive to everything that happened in the preceding hours: iron is highest in the morning and can fall by 30% or more by evening; a meal or an iron tablet raises it for several hours; an infection or inflammation pushes it down within a day as the body sequesters iron. That volatility is why clinicians rarely act on serum iron alone.

The companion tests turn it into something interpretable. Total iron-binding capacity (TIBC) measures how much transferrin is available to carry iron; transferrin saturation is serum iron divided by TIBC, expressed as a percentage — the proportion of carrier sites actually filled; and ferritin reflects the stored iron. A serum iron ordered by itself is usually an incomplete question.

A low serum iron on its own often causes nothing noticeable; symptoms usually arrive once iron stores are depleted enough to lower hemoglobin — iron-deficiency anemia. The classic picture:

High iron — iron overload, as in hereditary hemochromatosis or after many transfusions — builds slowly and is often silent for years. When symptoms come they include joint pain (classically the knuckles of the index and middle fingers), fatigue, abdominal pain and loss of libido, and in advanced disease liver damage, diabetes, heart problems and a bronze-gray skin tone.

Serum iron swings with meals and the time of day, so a single low or high value is not a diagnosis. Ferritin, TIBC and transferrin saturation together show whether stores are low or high, and a CBC shows whether anemia has followed.

Representative adult reference ranges are 65–175 mcg/dL for men and 50–170 mcg/dL for women, with the laboratory's range governing. Transferrin saturation is normally about 20–50%; TIBC about 250–450 mcg/dL; ferritin ranges vary by sex and lab but roughly 30–400 ng/mL in men and 15–150 ng/mL in premenopausal women, with the lower end debated because levels under 30 ng/mL already indicate depleted stores in most people.

A genuinely low serum iron, confirmed by a low ferritin or a low saturation, comes down to losing more iron than is absorbed. Ordered from most to least common in adults:

A high serum iron most often reflects timing — an iron supplement or an iron-rich meal in the previous hours, or a morning draw at the daily peak. A high value that repeats on a fasting morning sample, particularly with a transferrin saturation above 45%, is the screening signal for iron overload:

First, decide whether the result is real. An isolated serum iron that is high or low without matching ferritin and saturation is often a timing artifact and is repeated fasting, in the morning, at least 24 hours after any iron supplement. If the full panel shows iron deficiency, the questions are why the iron is being lost and how to replace it: menstrual history, gut symptoms, diet, medications and donation history point to the cause, and men and postmenopausal women with unexplained deficiency are referred for gastrointestinal evaluation. Replacement — oral or intravenous — is chosen by the clinician based on severity and absorption, and ferritin is rechecked after about three months.

If the panel shows the anemia-of-inflammation pattern, the target is the underlying condition rather than iron. If it shows overload — saturation above 45% with a raised ferritin — the next step is HFE genetic testing and an assessment of liver health; hemochromatosis is straightforward to treat when caught early, and untreated iron overload damages the liver, heart, pancreas and joints over decades.

Order the panel, not the single test: TestWell's iron panel bundles serum iron, TIBC, saturation and ferritin from one draw, and a CBC alongside shows whether any deficiency has progressed to anemia. Draw it in the morning after an overnight fast, skip iron supplements and multivitamins with iron for at least 24 hours beforehand (longer, if a clinician agrees, when the goal is to see your true baseline), and avoid testing during or shortly after an infection or illness, which suppresses serum iron and raises ferritin. Results post within a few business days with the laboratory's ranges; blood donors should wait a few weeks after donating for a representative result.