Summary
Serum iron measures the amount of iron circulating in the bloodstream at the time of the test. Although iron is essential for making haemoglobin, producing energy, and supporting immune function, serum iron alone is rarely sufficient to assess iron status — it fluctuates widely throughout the day and must be interpreted alongside ferritin and transferrin for a meaningful picture.
Iron deficiency is one of the most common nutritional deficiencies worldwide, particularly in women of reproductive age, pregnant women, vegetarians, and athletes. Serum iron is most useful when combined with ferritin (total iron stores) and transferrin (the carrier protein), allowing calculation of transferrin saturation — the most sensitive combined indicator of iron status.nnHigh serum iron can indicate iron overload (haemochromatosis), liver disease releasing stored iron, or haemolytic conditions where iron is released from red blood cells.
What It Is
Iron in the body serves two primary functions: it is incorporated into haemoglobin in red blood cells (where it binds oxygen) and into myoglobin in muscle cells (an oxygen reservoir). Iron also forms the active site of many enzymes involved in energy production and DNA synthesis.nnIn the bloodstream, iron circulates bound to transferrin — a carrier glycoprotein that transports iron from absorption sites in the gut and storage sites to bone marrow for red blood cell production. Serum iron measures the transferrin-bound iron concentration at the moment of testing.nnBecause serum iron has marked diurnal variation (highest in the morning, lowest in the evening) and is affected by recent dietary intake, inflammation, and infection, it has limited diagnostic value in isolation. The combination of serum iron + ferritin + transferrin (or TIBC) provides the most accurate picture of iron status.
Functions
Oxygen transport
Incorporated into haemoglobin within red blood cells, where it binds and carries oxygen from the lungs to all tissues.
Cellular energy production
Forms the active centres of mitochondrial cytochrome enzymes critical for ATP production in every cell.
Immune function
Required for the proliferation and activity of immune cells, particularly lymphocytes and natural killer cells.
Cognitive function and neurotransmission
Iron is essential for myelin synthesis and the production of dopamine, serotonin, and other neurotransmitters.
Reference Ranges
Serum Iron
Measured in µmol/L| Status | Range (µmol/L) | What it means |
|---|---|---|
| Low | < 11 | Low circulating iron — likely iron deficiency, especially if ferritin is also low. |
| Borderline | 11–15 | Lower end of range — interpret alongside ferritin and transferrin saturation. |
| Normal | 15–30 | Adequate circulating iron — normal haemoglobin production supported. |
| Elevated | > 30 | High circulating iron — investigate for iron overload, haemolysis, or liver disease. |
Serum iron varies significantly with the time of day, recent iron intake, and infection. Results must always be interpreted alongside ferritin, transferrin, TIBC, and CRP for a meaningful assessment of iron status.
Symptoms of Imbalance
Iron deficiency symptoms relate to impaired oxygen delivery and energy production; iron excess reflects organ damage from overload.
- Fatigue and weakness out of proportion to activity
- Pale skin, inner eyelids, and nail beds
- Shortness of breath on exertion
- Palpitations and light-headedness
- Cold hands and feet
- Brain fog, difficulty concentrating
- Brittle, spoon-shaped nails (koilonychia) in severe deficiency
- Joint pain and stiffness (particularly knuckles)
- Fatigue (paradoxically, also seen in iron overload)
- Abdominal pain
- Liver enlargement and tenderness
- Darkening of skin (bronze skin in haemochromatosis)
- Loss of libido and sexual dysfunction
- Cardiac arrhythmias (in severe overload)
Causes of Imbalance
- Inadequate dietary intake (vegetarian and vegan diets, restricted diets)
- Heavy menstrual periods — the leading cause in women
- Pregnancy and breastfeeding (increased demand)
- Gastrointestinal blood loss (ulcers, colorectal cancer, IBD)
- Coeliac disease or other malabsorption conditions
- Chronic kidney disease
- Recent blood donation or surgery
- Hereditary haemochromatosis (genetic iron overload disorder)
- Multiple blood transfusions
- Haemolytic anaemia (iron released from destroyed red cells)
- Acute liver disease (releases stored iron)
- Excessive iron supplementation
- Sideroblastic anaemia
FAQs
Serum iron fluctuates throughout the day, after meals, and during infections. Ferritin reflects the body’s total iron stores and is a far more stable and reliable indicator of deficiency — it starts to fall before serum iron, haemoglobin, or red cell indices change. A low ferritin confirms iron deficiency even if serum iron is still within range.
Yes. Iron deficiency has three stages: depleted stores (low ferritin only), iron-deficient erythropoiesis (low iron, raised TIBC, normal haemoglobin), and overt iron deficiency anaemia (low haemoglobin). Symptoms — fatigue, brain fog, restless legs — can occur at any stage, including before anaemia develops. Measuring ferritin identifies deficiency at the earliest stage.
Not ideal. Taking iron when levels are already adequate or elevated can be harmful — excess iron accumulates in organs. If you suspect iron deficiency, test first with serum iron and ferritin. If deficiency is confirmed, supplementation can be guided by the severity and cause of the deficiency.
Serum iron follows a strong diurnal rhythm, peaking in the morning (9–10am) and falling to its lowest in the evening and night — a variation of up to 30–40%. Testing in the morning before eating provides the most consistent and reproducible result for comparison between visits.
Recent iron supplementation or a high-iron meal before the test can temporarily elevate serum iron. Haemolysis (red cell breakdown in the sample tube) releases haemoglobin-bound iron and falsely elevates results. Liver disease releases stored iron into the circulation. CRP should be checked alongside iron markers to detect acute-phase responses that mask the true iron status.
References
- WHO. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. 2011. View source
- Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843. View source
- Lopez A, et al. Iron deficiency anaemia. Lancet. 2016;387(10021):907–916. View source