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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.
Polycythemia vera is a blood cancer that begins in the marrow of your bones, the soft center where new blood cells grow. With polycythemia vera, your marrow makes too many red blood cells, which causes your blood to get too thick. That can make you more likely to have blood clots, a stroke, or a heart attack.