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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. This slow-developing disease occurs when bone marrow produces too many red blood cells, which can lead to thickening of the blood. The condition is thought to be caused in part by a mutation in a gene called JAK2, which regulates the production of various types of blood cells, keeping them in proper balance.