What illness makes your blood thick? Understanding polycythemia vera

September 27, 2025 •

5 min read

While not widely known, a rare blood cancer called polycythemia vera is the primary illness that makes your blood thick by causing the bone marrow to overproduce red blood cells. Understanding this condition is crucial for managing symptoms and preventing complications like dangerous blood clots.

Quick Summary

Polycythemia vera is a rare blood cancer causing the bone marrow to produce too many red blood cells, leading to thick, slow-flowing blood that increases the risk of blood clots, strokes, and other serious health issues.

Key Points

Primary Cause: Polycythemia vera (PV), a rare blood cancer, is the main illness that makes your blood thick due to an overproduction of red blood cells.
Common Symptoms: Signs include headaches, dizziness, fatigue, itchy skin, and an enlarged spleen.
Serious Complications: Thickened blood can lead to dangerous blood clots, increasing the risk of heart attack, stroke, and pulmonary embolism.
Other Factors: Conditions like dehydration, sleep apnea, and Antiphospholipid Syndrome can also contribute to thick or hypercoagulable blood.
Effective Treatment: Management often involves therapeutic phlebotomy (blood removal), low-dose aspirin, and, in some cases, medication to control blood cell counts.
Diagnosis: A high blood cell count on a routine CBC, coupled with specific genetic testing (JAK2 mutation), helps confirm a PV diagnosis.

What is Polycythemia Vera?

Polycythemia vera (PV) is a chronic, progressive myeloproliferative neoplasm (MPN) where the bone marrow produces an excessive number of red blood cells. This overproduction can also involve white blood cells and platelets. The increase in blood cell volume and quantity, known as erythrocytosis, thickens the blood and slows its flow. The resulting condition, called hyperviscosity, can lead to serious health problems like blood clots, heart attack, and stroke.

Unlike an inherited disorder, PV is caused by an acquired genetic mutation, most commonly the JAK2 gene mutation. This mutation causes a protein to stay "on," continuously stimulating the production of blood cells in the bone marrow. While there is currently no cure, effective treatments can manage symptoms and reduce the risk of life-threatening complications, allowing many individuals to live a normal lifespan.

The Symptoms and Progression of Thick Blood

In its early stages, many people with PV experience no symptoms and the condition is often discovered during a routine blood test. As the disease progresses, a range of symptoms may appear as a result of the thicker blood slowing circulation and causing poor oxygen flow throughout the body.

Common Symptoms of Polycythemia Vera

Fatigue and Weakness: A very common symptom, extreme tiredness can significantly impact daily life.
Headaches and Dizziness: Reduced blood flow to the brain can cause headaches and a feeling of lightheadedness.
Vision Problems: Blurred or double vision and blind spots can occur due to circulation issues affecting the eyes.
Itchy Skin (Pruritus): A bothersome itch, especially after a warm bath or shower, is a hallmark symptom.
Enlarged Spleen (Splenomegaly): The spleen works harder to filter the extra blood cells, causing it to swell. This can cause pain or a feeling of fullness in the upper left abdomen.
Gout: The rapid turnover of blood cells can increase uric acid levels, leading to painful and swollen joints.
Reddened Skin (Plethora): An excess of red blood cells can give the skin, particularly on the face, a flushed or reddish appearance.
Numbness, Tingling, or Burning: A painful tingling or burning sensation in the hands and feet (erythromelalgia) can be a sign of poor circulation.

Complications of Thick Blood

If not properly treated, the thickened blood increases the risk of severe complications, most notably blood clots. These clots can form in various locations, leading to dangerous events such as:

Stroke: A blood clot blocking a vessel in the brain.
Heart Attack: A blood clot blocking an artery leading to the heart.
Pulmonary Embolism: A clot that travels to the lungs.
Deep Vein Thrombosis (DVT): A clot that forms in a deep vein, often in the legs.

Diagnostic Process for Polycythemia Vera

Diagnosing PV involves a combination of tests to confirm an abnormally high blood cell count and identify the underlying cause.

Complete Blood Count (CBC): A routine blood test can reveal a high red blood cell count (high hematocrit level), often along with an increase in white blood cells and platelets.
Genetic Testing: Testing for the JAK2 gene mutation is a crucial step, as over 95% of PV patients test positive for this genetic change.
Bone Marrow Biopsy: A sample of bone marrow is examined to confirm an overproduction of blood cells and rule out other bone marrow disorders.
Erythropoietin (EPO) Level Test: Erythropoietin is a hormone that stimulates red blood cell production. PV patients typically have a low EPO level because the bone marrow is overproducing cells independently.

Other Conditions that Can Cause Thick Blood

While PV is a direct illness that makes your blood thick by overproducing cells, other medical conditions can lead to hyperviscosity or a hypercoagulable state.

Secondary Polycythemia: Caused by external factors that trigger the body to produce more red blood cells in response to low oxygen levels (hypoxia). This can occur with conditions like sleep apnea, chronic obstructive pulmonary disease (COPD), or living at high altitude.
Antiphospholipid Syndrome (APS): An autoimmune disorder, also known as Hughes syndrome, where the immune system creates abnormal antibodies that cause blood to clot too easily.
Dehydration: When the body is dehydrated, the proportion of plasma (the liquid part of blood) decreases, making the concentration of blood cells higher and the blood appear thicker.
Certain Cancers: Some cancers can increase the risk of hypercoagulability, making blood clots more likely.

Management and Treatment of Polycythemia Vera

The goal of PV treatment is to reduce the number of excess blood cells, thin the blood, and prevent blood clots. A hematologist will tailor a treatment plan to the individual's needs.

Phlebotomy: The most common treatment involves regularly removing a pint of blood, similar to a blood donation, to reduce the red blood cell count and overall blood volume.
Low-Dose Aspirin: This medication is often prescribed to help prevent blood platelets from clumping together and forming clots.
Cytoreductive Therapy: For high-risk patients or those with bothersome symptoms, medications like hydroxyurea or interferon may be used to suppress bone marrow activity and lower blood cell counts.
Lifestyle Adjustments: Staying hydrated, getting moderate exercise to promote circulation, and avoiding smoking can help manage the condition.

Comparison: Primary vs. Secondary Polycythemia


Feature	Primary Polycythemia (Polycythemia Vera)	Secondary Polycythemia
Cause	Genetic mutation (e.g., JAK2) causing overproduction of blood cells in the bone marrow.	Low oxygen levels (hypoxia) trigger the body to produce more red blood cells.
EPO Level	Low, as the bone marrow is overproducing cells without external signaling.	High, as the body attempts to compensate for low oxygen by stimulating red blood cell production.
Other Cell Counts	Often elevated white blood cell and platelet counts.	Typically only red blood cells are elevated.
Underlying Issue	A problem within the bone marrow itself.	A compensatory response to an external factor or underlying health condition.
Treatment Focus	Reducing blood cell counts and managing bone marrow activity.	Treating the underlying condition that is causing the low oxygen levels.

Conclusion

Learning about what illness makes your blood thick is the first step toward effective management and care. Polycythemia vera is the most direct cause, leading to an overabundance of blood cells that thickens the blood. The long-term risks associated with this condition, particularly blood clots, emphasize the importance of early diagnosis and a consistent treatment plan. Fortunately, modern medical approaches and lifestyle adjustments allow individuals with PV and other related conditions to manage their health effectively and reduce complications.

For more detailed information on Polycythemia Vera, consult a trusted medical resource like the Mayo Clinic's guide to the disease.

Frequently Asked Questions

Thick blood, or hyperviscosity, is often first detected through a routine complete blood count (CBC) test, which will show an elevated hematocrit level. Further diagnosis, especially for polycythemia vera, may involve genetic testing for the JAK2 mutation and a bone marrow biopsy.

Early symptoms can be mild or non-existent, but common first signs can include headaches, dizziness, and a feeling of fatigue. Some people also notice red-flushed skin or an unusual itchiness, particularly after bathing.

Yes, dehydration can cause blood to become more concentrated. When there is less fluid (plasma) in your bloodstream, the concentration of red blood cells appears higher. This is a form of 'relative polycythemia' but is not caused by an overproduction of cells like polycythemia vera.

Polycythemia specifically refers to a high concentration of red blood cells, which physically thickens the blood. Hypercoagulability is a broader term for any condition that increases the tendency of blood to clot, which can be caused by thick blood but also by other factors like autoimmune disorders or genetics.

No, polycythemia vera is a chronic condition for which there is currently no cure. However, with appropriate medical treatment and regular monitoring, the disease can be effectively managed to control symptoms and prevent serious complications.

The most common treatments for PV include therapeutic phlebotomy to remove excess blood and reduce cell counts, and low-dose aspirin to prevent blood clots. More severe cases may require medication, such as hydroxyurea, to suppress bone marrow activity.

Severe sleep apnea can cause intermittent periods of low oxygen saturation during sleep. In response to this chronic low oxygen state, the body signals the bone marrow to produce more red blood cells in an attempt to carry more oxygen, leading to secondary polycythemia.