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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.
However, if the hematocrit gets too high (above 60 percent), it thickens the blood, causing circulatory difficulties that may result in strokes and heart problems. To avoid these problems, the patient can have phlebotomies (blood-letting) to reduce the hematocrit.