Iron is fundamental to human physiology. The body incorporates iron into hemoglobin, a protein responsible for binding oxygen in the lungs and transporting it to every tissue and organ. When iron levels fall too low, the production of healthy hemoglobin is impaired, leading to anemia. Anemia is a reduction in the blood’s capacity to carry oxygen. This lack of proper oxygen transport causes symptoms like fatigue, weakness, and shortness of breath, signaling that the body’s iron status requires professional assessment.
Key Blood Markers Used to Assess Iron Status
Determining iron status involves laboratory testing of several distinct markers, as iron exists in both functional and storage forms. The most common measure for diagnosing anemia is the hemoglobin (Hb) test, which measures the concentration of the oxygen-carrying protein in the blood. A low Hb value indicates the body is producing fewer or smaller red blood cells, confirming the anemic state.
Serum ferritin serves as a marker for the body’s iron stores. Ferritin is a protein that stores iron, primarily in the bone marrow, liver, and spleen. Because the amount of circulating ferritin reflects the size of these reserves, low ferritin is the earliest indicator that the body’s overall iron supply is being depleted.
Transferrin Saturation (TSAT) assesses the amount of iron actively being transported through the bloodstream. Iron is carried from storage sites to the bone marrow—where new red blood cells are produced—by transferrin. TSAT is calculated as a percentage, indicating how many of transferrin’s binding sites are occupied by iron. A low TSAT signifies that the iron supply available for immediate red blood cell production is limited.
Diagnostic Thresholds Defining Anemia
Anemia is defined by specific numerical thresholds for hemoglobin, which vary based on age and sex. For adult men, a hemoglobin level below 13.0 grams per deciliter (g/dL) meets the clinical definition of anemia. For non-pregnant adult women, the threshold is 12.0 g/dL.
To confirm the cause is iron deficiency, further testing is required. The most accepted threshold is a low serum ferritin level, reflecting depleted iron stores. A ferritin level below 30 nanograms per milliliter (ng/mL) is often used to diagnose low iron stores, even if hemoglobin is still in the normal range.
When inflammation is present, ferritin can be misleadingly elevated because it is an acute-phase reactant protein. In such cases, a lower ferritin cutoff, sometimes below 15 ng/mL, may be necessary to confirm iron deficiency. A Transferrin Saturation (TSAT) value below 16 percent also indicates inadequate iron supply for red blood cell synthesis.
The Stages of Iron Depletion Leading to Anemia
Iron deficiency is a gradual process that develops across three recognized stages. The first is Iron Stores Depletion, which occurs when iron intake does not meet daily needs, forcing the body to draw down its reserves. During this phase, the serum ferritin level drops, but the hemoglobin concentration remains normal.
The second stage is Iron-Deficient Erythropoiesis. This begins when iron stores are so low that they impair the bone marrow’s ability to produce healthy red blood cells. The Transferrin Saturation (TSAT) falls below the threshold, indicating an insufficient iron supply for new blood cell production.
The final stage is Iron-Deficiency Anemia. This is reached when the deficit of functional iron causes the hemoglobin concentration to drop below established clinical thresholds. The body’s oxygen-carrying capacity is noticeably reduced, and the patient typically experiences physical symptoms of anemia, such as extreme fatigue.
Primary Reasons Iron Levels Decline
The most common reason for iron levels to decline in adults is chronic blood loss, which causes the body to lose iron faster than it can be replenished. For premenopausal women, heavy menstrual bleeding is the primary cause of iron deficiency. For men and postmenopausal women, blood loss is often linked to the gastrointestinal tract, potentially due to ulcers, polyps, or other conditions that cause slow, persistent bleeding.
Inadequate dietary intake is another factor, especially for individuals following vegetarian or vegan diets, as non-meat iron sources are less efficiently absorbed. Increased demand for iron, such as during pregnancy or periods of rapid growth, can also lead to deficiency if the diet is not adjusted.
Impaired iron absorption is a third major cause, even when the diet contains sufficient iron. Conditions like celiac disease, inflammatory bowel disease, or prior bariatric surgery can damage the small intestine where iron is absorbed. Additionally, frequent use of antacid medications reduces stomach acid, which is necessary to convert iron into an absorbable form.