Thick Blood (High Blood Viscosity): Symptoms, Causes and Tests for Men

Short answer: “Thick blood” usually means high blood viscosity: there are too many red blood cells, or too little plasma, so blood flows less easily. The most common cause of a high hematocrit is simply dehydration, but smoking, living at high altitude, obstructive sleep apnea, heart or lung disease, testosterone replacement therapy and a bone marrow disorder called polycythemia vera can also raise it. Thick blood often causes no symptoms at all, so the only reliable way to know is a complete blood count (CBC) ordered by your doctor.

Key Takeaways

• Blood viscosity describes how thick your blood is. In everyday checkups it is estimated through hematocrit and hemoglobin, which are part of a routine CBC.
• MedlinePlus names dehydration as the most common cause of a high hematocrit. Drinking fluids and retesting often settles the question.
• Mayo Clinic lists smoking and high altitude as the most common causes of a high hemoglobin count, with sleep apnea, heart and lung disease and testosterone therapy among the others.
• Polycythemia vera is rare but serious: it thickens the blood and raises the risk of clots, heart attack and stroke. It needs a hematologist, not a supplement.
• Headache, dizziness, blurred vision, itching after a warm shower or one swollen, painful leg are reasons to see a doctor promptly.
• No herb or supplement is a proven way to “thin” blood. Panax ginseng may interfere with blood clotting, so men on blood thinners should ask their doctor first.

What Is Blood Viscosity (Thick Blood)?

Blood is a mix of cells floating in plasma, a liquid that is mostly water. Viscosity is a measure of how much that mix resists flowing. Honey has high viscosity, water has low viscosity. When the share of red blood cells goes up, or the plasma volume goes down, blood behaves a little more like honey: the heart has to push harder, and flow through the smallest vessels slows.

Doctors rarely measure viscosity directly. Instead they look at hematocrit, the percentage of your blood made up of red blood cells. MedlinePlus explains that a hematocrit result of 42 means red cells make up 42 percent of your blood^[1]. Hemoglobin, the oxygen carrying protein inside red cells, usually moves in the same direction. Both appear on a standard CBC.

People also use “thick blood” loosely to mean blood that clots too easily. That is a different problem. Clotting disorders involve clotting proteins and platelets rather than the number of red cells, and they are checked with different tests. If a relative has had unexplained clots, tell your doctor, because the workup is not the same.

What Causes Thick Blood?

Causes fall into two broad groups: too little plasma (the red cells are concentrated) and too many red cells (the body or the bone marrow is making extra).

Too little plasma: dehydration

MedlinePlus calls dehydration “the most common cause of a high hematocrit”^[1]. Heavy sweating, a hot day, a stomach bug, alcohol or a morning blood draw after a night without fluids can all nudge the number up. The red cell count itself has not changed, so the value usually returns to normal once you are rehydrated. Shock, a medical emergency, can also reduce plasma volume^[1].

Too many red cells: the body responding to low oxygen

When tissues sense less oxygen, the body makes more red blood cells to compensate. That is why Mayo Clinic lists smoking and living at high altitude as the most common causes of a high hemoglobin count^[2]. The same logic applies to heart and lung conditions such as COPD, emphysema, heart failure and congenital heart disease, and to obstructive sleep apnea, where breathing pauses during sleep lower oxygen levels again and again^[2]. If you snore loudly and wake up tired, our article on sleep apnea and testosterone explains why screening is worth it.

Medicines and hormones: testosterone therapy

Mayo Clinic notes that testosterone replacement therapy can raise hemoglobin as a side effect^[2]. This is one of the main reasons men on testosterone are asked to repeat blood counts during treatment. A rising hematocrit on TRT is not something to manage on your own: your prescriber may adjust the dose, the form of testosterone or the schedule. Never stop or change a prescription without talking to the person who wrote it.

Bone marrow overproduction: polycythemia vera

Polycythemia vera (PV) is a rare blood disorder in which the bone marrow makes too many red blood cells, so the blood becomes thick and flows poorly^[3]. Cleveland Clinic and the National Heart, Lung, and Blood Institute both describe it as a type of blood cancer within the myeloproliferative neoplasms^[3][5]. It is usually linked to an acquired change in the JAK2 gene that is not inherited^[4]. MedlinePlus notes that PV occurs more often in men and is usually not seen in people under 40^[4], and Cleveland Clinic says most people are in their 60s before symptoms appear^[3].

Other conditions

Less often, a high hemoglobin count is linked to kidney cancer or liver cancer^[2]. This is one reason a doctor will not simply assume dehydration if a high result keeps coming back.

Thick Blood Symptoms

Mildly high hematocrit, especially from dehydration, usually causes no symptoms at all. When blood is markedly thick, as in polycythemia vera, MedlinePlus and Cleveland Clinic describe symptoms such as^[3][4]:

• Headaches, dizziness and unusual tiredness
• Blurred or double vision
• Ringing in the ears
• Itching, especially after a warm bath or shower
• Reddened skin, particularly on the face
• Shortness of breath, including when lying down
• Burning or tingling in the hands and feet, and joint pain
• A feeling of fullness in the upper left abdomen from an enlarged spleen
• Easy bruising, nosebleeds or signs of a clot in a surface vein

Many of these overlap with poor circulation in general. If cold hands and feet, leg cramps when walking or slow healing sound familiar, read our guide to the signs of poor circulation, which covers the vessel side of the picture.

Why it matters: clots

The real danger of very thick blood is clotting. Cleveland Clinic states that polycythemia vera raises the risk of blood clots, heart attacks and strokes^[3]. Treatment in PV aims to reduce blood thickness, and Cleveland Clinic notes that hematocrit is usually kept below 45 percent^[3].

How Thick Blood Is Tested

The starting point is a complete blood count, a simple blood draw from a vein in the arm that MedlinePlus says usually takes less than five minutes and normally needs no special preparation^[1]. Normal ranges vary with sex, age, smoking status and altitude, so ask your clinician what your target should be rather than comparing to a number online^[1]. Our article on health checkups for men lists the routine tests worth asking about.

If the result is high and does not settle after rehydration, a doctor may order^[3][4]:

• Repeat CBC with differential to see whether white cells and platelets are also high
• Erythropoietin (EPO) level: low levels point toward PV, high levels suggest a secondary cause such as low oxygen
• Blood oxygen saturation and, if apnea is suspected, a sleep study
• JAK2 gene test for polycythemia vera
• Bone marrow biopsy in selected cases

Treatment and Everyday Habits

Treatment depends entirely on the cause. Dehydration is corrected with fluids. Secondary causes are handled by treating the source: stopping smoking, treating sleep apnea, managing lung or heart disease, or adjusting testosterone therapy. For polycythemia vera, MedlinePlus and Cleveland Clinic describe therapeutic phlebotomy (removing blood, about one unit weekly at first until counts fall), medicines such as hydroxyurea, interferons or ruxolitinib, and aspirin for some patients^[3][4]. Note that MedlinePlus warns aspirin also raises the risk of stomach bleeding^[4], which is exactly why these decisions belong with a specialist.

Habits that support overall heart and vessel health still matter, and Cleveland Clinic lists several for people with PV: stay active, do not smoke, avoid high altitudes, manage your weight and monitor blood pressure^[3]. Drinking enough fluid through the day, especially in heat or during exercise, keeps a dehydrated blood test from giving a misleading result.

What does not belong on the list: self-treating with “blood thinning” herbs, fish oil megadoses or aspirin you started on your own. These can add to the effect of prescribed anticoagulants and raise bleeding risk.

Supplements, Blood Thinners and Ultra Bann

No herb or supplement has been shown to safely lower hematocrit or thin the blood, and you should not use any product for that purpose. Some herbs can actually make medication management harder. NCCIH states that Asian ginseng (Panax ginseng) “may worsen autoimmune disorders and interfere with blood clotting,” that it might lower blood sugar, and that it might interact with certain medications^[6]. NCCIH also notes that short term use, up to 6 months, appears safe for most people, while long term safety is less clear^[6]. More detail is in our Panax ginseng guide.

This matters for anyone considering Ultra Bann, a male vitality supplement by DL Bioproducts taken as one capsule every 2 days. Each serving contains 110 mg of Panax ginseng alongside six other botanicals, with every amount listed on the Ultra Bann ingredients label. Ultra Bann is not designed to change blood thickness or circulation and should not be used for that. If you take warfarin or another blood thinner, have polycythemia, a clotting disorder, diabetes or upcoming surgery, show the label to your doctor or pharmacist before taking it.

Frequently Asked Questions

What are the first signs of thick blood?

Often there are none. When symptoms do appear, the common ones are headache, dizziness, tiredness, blurred vision and itching after a warm shower^[3][4]. Because these are vague, a CBC is the only way to know.

Can dehydration make your blood thick?

Yes. Dehydration lowers plasma volume, so red cells make up a larger share of the blood. MedlinePlus calls it the most common cause of a high hematocrit^[1]. A repeat test after drinking normally often comes back lower.

Does testosterone therapy thicken the blood?

It can. Mayo Clinic lists testosterone replacement therapy as a cause of a high hemoglobin count^[2]. Men on TRT should keep their scheduled blood tests and let the prescriber decide on any change.

Is thick blood the same as polycythemia?

Not always. Polycythemia means too many red blood cells, which does thicken the blood. But a high hematocrit can also come from dehydration, where the red cell count is normal and plasma is low^[1]. Polycythemia vera is one specific, rare bone marrow form^[4].

When should I see a doctor about thick blood?

See a doctor if a blood test shows a high hematocrit or hemoglobin that does not settle after rehydration, or if you have frequent headaches, vision changes or itching after showers. Seek urgent care for one swollen, painful leg, chest pain, sudden shortness of breath, or stroke signs such as face drooping or weakness on one side.

References

1. MedlinePlus, U.S. National Library of Medicine. Hematocrit Test.
2. Mayo Clinic. High hemoglobin count: Causes.
3. Cleveland Clinic. Polycythemia Vera.
4. MedlinePlus Medical Encyclopedia. Polycythemia vera.
5. National Heart, Lung, and Blood Institute. Polycythemia Vera.
6. National Center for Complementary and Integrative Health. Asian Ginseng.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease. This article is for information only and is not medical advice.