Understanding Microcytosis: Causes, Symptoms, and Management

Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026

Microcytosis means your red blood cells are smaller than normal, which a routine blood count shows as a mean corpuscular volume (MCV) below 80 femtoliters. It is a description rather than a diagnosis, and it most often points to iron deficiency [1].

Quick Answer: What is microcytosis?

Microcytosis describes red blood cells that are smaller than normal, found on a complete blood count when MCV is below 80 fL in adults. It is not a diagnosis itself but a clue, most often to iron deficiency and sometimes to thalassemia or long-term illness. Many people have no symptoms, and it often reverses once the cause is treated. [1]

What the research says

• Iron deficiency and thalassemia trait account for most microcytosis. In a study of 925 blood donor samples in India, 50 (5.4%) were microcytic; of those, 52% had iron deficiency anemia, 36% had beta-thalassemia trait, 8% had both, and 4% remained undiagnosed (Tiwari et al., Transfus Med, 2010 [2]).

• Ferritin is the first test to order. A clinical review defines adult microcytosis as an MCV below 80 fL, names iron deficiency anemia and thalassemia trait as the most common causes, and recommends serum ferritin as the first laboratory test, since a low ferritin suggests iron deficiency; if ferritin is not low, further tests such as total iron-binding capacity, transferrin saturation, serum iron, and possibly hemoglobin electrophoresis follow (Van Vranken, Am Fam Physician, 2010 [3]).

• In adults, iron deficiency needs a cause. A clinical review reported that iron deficiency anemia affects about 2% of adult men, 9 to 12% of non-Hispanic white women, and nearly 20% of Black and Mexican-American women in the United States, and that 9% of patients older than 65 with iron deficiency anemia had a gastrointestinal cancer when evaluated, a reason the source of the deficiency should always be investigated (Killip et al., Am Fam Physician, 2007 [4]).

What is microcytosis, and why does it matter clinically?

Microcytosis means you have smaller-than-normal red blood cells. It is a description, not an official diagnosis, and you may learn about it after routine blood work called a complete blood count (CBC). One value on a CBC is the mean corpuscular volume (MCV), which is the average size of your red blood cells. For adults, microcytosis means the MCV is less than 80 fL [1].

Microcytosis is often a sign of microcytic anemia. In microcytic anemia, red blood cells do not have enough hemoglobin, the essential protein that carries oxygen from your lungs to the rest of your body. Microcytic anemia is the most common form of anemia in adults and children. Microcytosis itself is just a characteristic of your red blood cells and is often a harmless finding, but it can be one sign among others that a condition needs treatment [1].

What are the common causes of microcytosis?

Microcytosis most often occurs when your body is not getting enough iron, an essential ingredient in hemoglobin. Environmental factors, medical conditions (inherited and acquired), and certain behaviors can also lead to smaller red blood cells [1].

Cause	Description
Iron deficiency	The most common cause. It can result from too little iron in the diet, from conditions that block absorption (such as celiac disease, H. pylori infection, or gastric bypass surgery), or from blood loss such as heavy menstrual bleeding or gastrointestinal bleeding.
Thalassemia	An inherited disorder that disrupts the body's ability to make hemoglobin. The number of red blood cells may be normal or increased, but each cell has low hemoglobin. Mild types can cause microcytosis without anemia.
Anemia of chronic disease	Long-term conditions such as autoimmune diseases, infections, kidney disease, and cancer cause inflammation that disrupts how the body uses iron to make healthy red blood cells.
Less common causes	Lead poisoning, copper deficiency, and rare inherited genetic conditions that interfere with the production and function of red blood cells.

What are the symptoms and warning signs of microcytosis?

Often there are no symptoms, and the only sign of microcytosis is a low MCV value. When symptoms do occur, they are the same as those of microcytic anemia [1].

• Fatigue and low energy [1].

• Pale skin [1].

• Shortness of breath [1].

• A rapid heartbeat [1].

• Pica, cravings for non-food items such as ice or dirt, which occurs only when the microcytosis is caused by iron deficiency [1].

You may also notice symptoms related to whatever is causing the microcytosis [1].

A normal red blood cell compared with a smaller microcytic cell, and the most common symptoms when anemia is present.

How is microcytosis diagnosed?

Because microcytosis is not an official diagnosis, your provider considers your symptoms, age, sex, and medical history to work out the likely cause. For example, iron deficiency anemia is a likely cause if you have heavy periods, and a family history of thalassemia may point to thalassemia. Tests that help find the cause include [1].

1. Peripheral blood smear: a provider looks at your blood cells under a microscope to confirm that they are small and to spot other abnormalities [1].

2. Red cell distribution width (RDW): measures how much your red blood cells vary in size, which can suggest certain conditions [1].

3. Iron studies: iron levels, ferritin (the protein that stores iron), and total iron-binding capacity (TIBC) help diagnose iron deficiency and identify anemia of chronic disease [1].

4. Hemoglobin electrophoresis and genetic tests: used to confirm thalassemia once iron deficiency has been ruled out [1].

A low MCV is the starting point, and iron studies and ferritin help separate iron deficiency from thalassemia.

How is microcytosis treated and managed?

Treatment depends on what is causing your small red blood cells. You may not need treatment if you have no symptoms and your test results are not concerning. Depending on the cause, options include [1].

• Dietary changes: getting enough iron from foods such as tuna, tofu, peas, beans, and many fruits and vegetables [1].

• Iron supplements: for iron deficiency anemia, as directed by your provider [1].

• Blood transfusion: if the anemia is very severe [1].

• Treating the underlying condition: following your provider's plan for the issue behind the microcytosis, such as an autoimmune disease or thalassemia [1].

You cannot prevent every condition that leads to small red blood cells, but getting enough iron in your diet helps reduce your risk. If you have a condition that limits iron absorption, such as having had gastric bypass surgery, talk to your provider about which iron supplements you might need [1].

An iron-rich diet, supplements when recommended, and regular medical follow-up are the main pillars of managing microcytosis.

What is the long-term outlook for microcytosis?

Not everyone with microcytosis or microcytic anemia needs treatment. Often, treating the underlying issue reverses microcytosis, though this depends on the cause. If a serious condition is behind your small red blood cells, it is important to be diagnosed and treated as soon as possible, and some inherited causes such as thalassemia may need ongoing follow-up. Follow your provider's guidance about when treatment is needed and what outcomes to expect [1].

What is the bottom line on microcytosis?

Microcytosis is a common lab finding and a useful clue to your nutritional and blood health. Learning that your red blood cells are small can be worrying, but it most often points to a manageable problem such as iron deficiency. If a recent blood test showed a low MCV, or if you have persistent fatigue, schedule a follow-up with your doctor so the right tests can find the cause and guide treatment.

Frequently Asked Questions

What does it mean if my MCV is low?

A low MCV (mean corpuscular volume) means your red blood cells are smaller than normal, which is called microcytosis. For adults, that means an MCV below 80 fL. It is one piece of information that your provider interprets together with your other results.

Is microcytosis cancer?

Microcytosis itself is not a cancer. It is simply a description of small red blood cells. It can occasionally be a sign of cancer, but it is usually a sign of other conditions, most often iron deficiency, so it is important not to assume anything until your provider explains your results.

Can a poor diet cause small red blood cells?

Yes. Not getting enough iron from food can lead to iron deficiency, the most common cause of microcytosis. Problems absorbing iron and blood loss can also cause it.

How is microcytosis different from macrocytosis?

Microcytosis means red blood cells that are smaller than normal, while macrocytosis means red blood cells that are larger than normal.

What is the most common cause of microcytosis?

Iron deficiency is the most common cause. Thalassemia and anemia of chronic disease are also common causes.

Are there any symptoms of microcytosis?

Often there are none, and the only sign is a low MCV on a blood test. When symptoms occur, they include fatigue, pale skin, shortness of breath, and a rapid heartbeat.

Is microcytosis inherited?

Microcytosis can be. Thalassemia is an inherited disorder that affects how the body makes hemoglobin, and rare inherited genetic conditions can also cause small red blood cells.

How do doctors treat small red blood cells?

Treatment depends on the cause. It may include dietary changes, iron supplements for iron deficiency anemia, a blood transfusion for very severe anemia, or treatment of an underlying condition.

Can lead exposure cause microcytosis?

Yes. Lead poisoning is a less common cause of microcytosis.

Is microcytosis reversible?

It depends on the cause. Often, treating the underlying issue, such as iron deficiency, reverses microcytosis.

References

1. Cleveland Clinic. Microcytosis. Cleveland Clinic. June 21, 2023. https://my.clevelandclinic.org/health/diseases/25088-microcytosis

2. Tiwari AK, Chandola I, Ahuja A. Approach to blood donors with microcytosis. Transfus Med. 2010;20(2):88-94. PMID 20003131. https://doi.org/10.1111/j.1365-3148.2009.00980.x

3. Van Vranken M. Evaluation of microcytosis. Am Fam Physician. 2010;82(9):1117-1122. PMID 21121557. https://pubmed.ncbi.nlm.nih.gov/21121557/

4. Killip S, Bennett JM, Chambers MD. Iron deficiency anemia. Am Fam Physician. 2007;75(5):671-678. PMID 17375513. https://pubmed.ncbi.nlm.nih.gov/17375513/

Medical disclaimer

This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. The clinical information in this guide reflects source material last reviewed in June 2023.