In which condition is the bone marrow producing too many red blood cells, leading to thick blood?

The condition characterized by the overproduction of red blood cells by the bone marrow, resulting in thickened blood, is known as polycythemia vera. This is a myeloproliferative neoplasm where there is an abnormal increase in red blood cell mass, which can lead to increased blood viscosity and a range of complications, such as thrombosis and hypertension.

In polycythemia vera, the overproduction of red blood cells occurs independently of the body's oxygen demand or erythropoietin levels, often due to mutations in genes such as JAK2. This condition can be associated with symptoms like headaches, dizziness, vision changes, and can lead to more serious complications like blood clots.

In comparison, thalassemia involves a genetic defect affecting hemoglobin production but does not lead to increased red blood cell production per se; instead, it is characterized by abnormal hemoglobin synthesis. Sickle cell disease does lead to complications involving red blood cells but is caused by a mutation that leads to sickling of cells, resulting in pain and potential vascular obstruction. Iron deficiency anemia is a condition where there is insufficient hemoglobin due to a lack of iron, leading to a reduced red blood cell count, hence not related to excessive red