Overview
Iron is an essential mineral required for normal growth, development, and maintenance of vital physiological functions. Serum iron measurement reflects the amount of circulating iron bound to transferrin, the main iron transport protein in the blood. Iron plays a central role in hemoglobin synthesis, enabling red blood cells to transport oxygen from the lungs to all tissues of the body.
Iron is also a critical cofactor for many enzymes involved in metabolic pathways, cellular respiration, and energy production. When iron levels are inadequate, red blood cells fail to transport sufficient oxygen, leading to fatigue, weakness, and reduced physical capacity. Serum iron, when interpreted along with ferritin and transferrin parameters, provides valuable insight into iron status, utilization, and metabolism.
Role of Iron in the Body
Iron is a major structural component of hemoglobin and myoglobin. Hemoglobin carries oxygen in red blood cells, while myoglobin stores oxygen in muscle tissue. Adequate iron levels are therefore essential for oxygenation of tissues and maintenance of muscle function.
Beyond oxygen transport, iron is involved in converting blood glucose into energy through its role in mitochondrial enzymes. It supports immune function by enabling proper activity of immune cells and contributes to cognitive function by supporting neurotransmitter synthesis and brain metabolism. Iron also plays a role in maintaining healthy skin, hair, and nails, reflecting its importance in cellular growth and repair.
Forms and Distribution of Iron
Iron in the body exists in several forms. The majority of iron, approximately 65%, is present in hemoglobin within red blood cells. About 4% is found in myoglobin in muscle tissue.
Roughly 30% of body iron is stored in the form of ferritin or hemosiderin, mainly in the liver, spleen, and bone marrow. A very small fraction, around 1%, is present in other enzymes and proteins. Serum iron represents only a small circulating portion of total body iron, but is crucial for assessing iron availability.
Iron Metabolism and Transport
Dietary iron is absorbed mainly in the duodenum. After absorption, iron binds to transferrin in plasma and is transported to the bone marrow for erythropoiesis or to storage sites such as the liver.
Iron loss occurs through shedding of intestinal mucosa, skin, and minor bleeding. The body tightly regulates iron absorption, as there is no active mechanism for iron excretion. Disruption in absorption, transport, or storage leads to iron deficiency or overload states.
Indications for Serum Iron Testing
Serum iron testing is indicated in the evaluation of unexplained anemia. It is commonly used in suspected iron deficiency anemia and iron overload disorders such as hemochromatosis.
The test is useful in hemolytic anemia, liver tissue damage, hepatitis, and iron poisoning. It is also indicated in individuals with frequent blood transfusions and in patients undergoing iron therapy to monitor treatment response. Hair loss and chronic diseases affecting iron metabolism are additional indications for serum iron estimation.
Methods of Estimation
Serum iron can be estimated using immunoassay techniques, colorimetry, and spectrophotometry. These methods measure circulating iron bound to transferrin with high analytical accuracy.
Accurate estimation depends on proper sample handling and adherence to pre-analytical precautions, as iron levels are sensitive to external influences.
Sample Collection and Precautions
For serum iron estimation, 3.0 mL of blood is collected in a lithium heparin (green-capped) tube or a plain (red-capped) tube. Serum or plasma should be separated as early as possible after collection.
Patients are advised to stop iron supplements at least 24 hours before testing. Sample collection is preferably done in the morning or in a fasting state due to diurnal variation. Certain medications, such as antibiotics, oral contraceptives, estrogens, blood pressure drugs, cholesterol drugs, iron chelating agents, gout medications, and testosterone, may affect results and should be considered during interpretation. Grossly hemolyzed samples are rejected.
Reference Range
Normal serum iron reference ranges vary with age and sex. Newborns typically have higher iron levels, while adult males and females have different reference intervals.
Adult males generally show a reference range of approximately 45–182 µg/dL, while adult females range from about 28–170 µg/dL. These ranges may vary slightly depending on laboratory methods and population.
Interpretation of Low Serum Iron
Low serum iron levels indicate reduced circulating iron and are commonly seen in iron deficiency anemia. Chronic blood loss, poor dietary intake, pregnancy, and malabsorption syndromes are frequent causes.
Anemia of chronic disease is another important cause, where inflammation leads to low serum iron despite adequate or increased iron stores. In such cases, ferritin levels are often normal or elevated, highlighting the need for combined interpretation.
Interpretation of High Serum Iron
Elevated serum iron levels suggest iron overload. This may occur in hereditary hemochromatosis, hemolytic anemia, liver disease, hepatitis, iron poisoning, or repeated blood transfusions.
Increased serum iron may also be seen in liver tissue damage, where stored iron is released into circulation.
Factors Affecting Serum Iron Levels
Serum iron shows diurnal variation, with higher levels in the morning and lower levels later in the day. Inflammatory states can lower serum iron even when total body iron stores are adequate.
Recent blood transfusions, iron supplementation, and iron-rich meals can transiently increase serum iron levels. Medications may also influence results, making clinical correlation essential.
Clinical Utility and Limitations
Serum iron is a valuable component of iron studies, but should never be interpreted alone. It must be evaluated alongside ferritin, transferrin, total iron-binding capacity, and transferrin saturation.
Ferritin is more sensitive for diagnosing iron deficiency, while transferrin saturation provides insight into iron availability. Serum iron measures only circulating iron, which represents a small fraction of total body iron and fluctuates significantly throughout the day.
When interpreted correctly within a complete iron profile, serum iron plays an important role in diagnosing iron deficiency, iron overload, and disorders of iron metabolism.
