What is polycythemia?
Polycythemia is an abnormal increase in red blood cell production that can occur as a side effect of testosterone replacement therapy.
Polycythemia refers to an abnormal rise in the number of red blood cells circulating in the bloodstream. When testosterone levels are raised through TRT, the body may respond by producing excess red blood cells, which thickens the blood and raises hematocrit levels (the percentage of blood volume made up of red cells). This response occurs because testosterone stimulates erythropoietin production in the kidneys, which signals bone marrow to manufacture more red blood cells.
For men undergoing testosterone replacement therapy, polycythemia matters because thicker blood increases the risk of clotting, stroke, and cardiovascular events. Elevated hematocrit can also cause headaches, dizziness, and a flushed appearance. Most clinicians monitor hematocrit levels regularly during TRT with blood work and may recommend dose adjustments, blood donation, or temporary therapy pauses if polycythemia develops. The condition is dose-dependent, meaning higher testosterone doses carry greater risk.
Recognition of polycythemia as a known TRT side effect has made routine hematocrit testing standard practice among providers. Men starting or adjusting testosterone therapy should expect baseline and periodic follow-up labs to catch rising red cell counts early. Managing polycythemia proactively prevents serious complications and allows therapy to continue safely.