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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.
Too many red blood cells is referred to by doctors as polycythaemia and erythrocytosis.The main problem caused by polycythaemia is that the high number of red cells increases the blood's 'thickness' (viscosity).