Microcytosis is not a disease, but a technical term describing red blood cells that appear smaller than normal in laboratory measurements. This finding is common in routine blood work and indicates an underlying health issue affecting blood cell development. Red blood cells transport oxygen from the lungs to tissues throughout the body. A reduction in their size typically signals a problem with the production of hemoglobin, the protein that carries this oxygen. The presence of these abnormally small cells suggests the body’s oxygen delivery capacity is compromised, prompting further investigation.
How Red Blood Cell Size is Measured
The presence of microcytosis is determined by calculating the Mean Corpuscular Volume (MCV), a metric included in a standard Complete Blood Count (CBC) test. The MCV represents the average volume of a person’s red blood cells, measured in femtoliters (fL). For adults, a normal MCV value generally falls within the range of 80 to 100 fL.
A result below 80 fL is the laboratory definition of microcytosis, indicating that the circulating red blood cells are smaller than average. This measurement is a descriptive finding, not a diagnosis, that directs clinicians toward specific categories of anemia or blood disorders. The MCV helps differentiate between various causes of low hemoglobin.
The Primary Conditions Causing Microcytosis
The primary cause of microcytosis is an issue with the body’s ability to synthesize sufficient hemoglobin, often tied to iron availability or genetic defects.
Iron Deficiency Anemia (IDA)
The most common underlying condition is Iron Deficiency Anemia (IDA). A lack of iron directly limits the production of the oxygen-carrying heme component of hemoglobin. Without enough iron, red blood cell precursors undergo an additional division in the bone marrow, resulting in a smaller cell that contains less hemoglobin, often appearing pale (hypochromic).
Thalassemia
A second major cause is Thalassemia, an inherited disorder involving a genetic defect in the synthesis of the globin protein chains that form the hemoglobin molecule. This is a quantitative defect, meaning the body produces a reduced amount of structurally sound globin chains. The imbalance of globin chains results in the premature destruction of developing red blood cells and the release of smaller mature cells into the bloodstream, even when iron stores are adequate.
Anemia of Chronic Disease (ACD)
Anemia of Chronic Disease (ACD) can also lead to microcytosis, although this condition more frequently causes red cells of normal size. In ACD, chronic inflammation from conditions like autoimmune disease or cancer elevates levels of a hormone called hepcidin. Hepcidin effectively traps iron within the body’s storage cells, such as macrophages, making it functionally unavailable for new red blood cell production.
Sideroblastic Anemia
Another cause is Sideroblastic Anemia, a group of disorders where the body has sufficient iron but cannot properly incorporate it into the heme structure. The iron instead accumulates in the mitochondria of the red blood cell precursors, forming a characteristic ring around the nucleus. This inability to utilize iron for hemoglobin synthesis impairs red blood cell maturation, resulting in the release of smaller-than-normal cells into circulation.
Physical Symptoms and Associated Health Impacts
The physical signs associated with microcytosis are typically those of the underlying anemia, stemming from the reduced capacity of the small red blood cells to deliver oxygen. The most commonly reported symptom is persistent fatigue, which arises because the body’s tissues and organs are not receiving adequate oxygen. This lack of energy is often accompanied by generalized weakness and lethargy.
Patients may also notice pallor, or an unusual paleness of the skin and the inner lining of the eyelids, a direct consequence of the low hemoglobin concentration. When the oxygen supply is significantly reduced, individuals may experience shortness of breath, particularly during physical exertion. In severe cases, especially those caused by iron deficiency, an unusual craving for non-food items like ice or dirt, known as pica, can also develop.
If the underlying cause of microcytosis remains unaddressed, the sustained oxygen deprivation can place a significant strain on the cardiovascular system. The heart must pump harder and faster to compensate for the poor oxygen-carrying capacity of the blood, potentially leading to an accelerated heart rate. Long-term, this increased workload can contribute to heart-related issues.