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Thick blood is caused by a variety of conditions, such as heavy proteins in the blood, too much blood in circulation or a blood clot system imbalance, according to Southeastern Medical Oncology Center. Diseases such as cancer and lupus can also lead to thick blood. Continue Reading.
Excess Blood Cells. Certain diseases, namely blood cancers, thicken the blood because they lead to abnormally high levels of blood cells. While rare, one of the more common reasons for thick blood is called polycythemia vera (PV), in which the body produces excessive blood cells -- mostly too many red blood cells.