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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.
When the number of red blood cells is too low (anemia), blood carries less oxygen, and fatigue and weakness develop. When the number of red blood cells is too high (polycythemia), blood can become too thick, which may cause the blood to clot more easily and increase the risk of heart attacks and strokes.