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Exogenous testosterone supplementation can lead to polycythemia, a condition characterized by elevated levels of red blood cells. This occurs because testosterone stimulates the production of erythropoietin, a hormone produced by the kidneys that promotes the formation of red blood cells in the bone marrow. Increased erythropoietin levels, in turn, stimulate the production of more red blood cells, raising the hematocrit and hemoglobin levels in the blood.
This increase in red blood cell mass can make the blood more viscous, potentially leading to complications such as clot formation, strokes, or heart attacks. It's important for individuals using testosterone replacement therapy to have regular medical check-ups, including blood tests to monitor for polycythemia.