What is erythrocytosis?
Erythrocytosis is an abnormal increase in the number of red blood cells in the body, a potential side effect of testosterone replacement therapy that can affect oxygen transport and blood viscosity.
Erythrocytosis refers to an elevated concentration of red blood cells (erythrocytes) in the blood, a condition that can develop in patients using testosterone replacement therapy. When TRT increases testosterone levels, it stimulates the bone marrow to produce more red blood cells, raising hematocrit levels above normal ranges (typically above 50-54% depending on lab standards).
This side effect occurs because testosterone promotes erythropoiesis, the production of red blood cells. While moderate red blood cell elevation may not cause immediate symptoms, sustained erythrocytosis increases blood viscosity, which means the blood becomes thicker and flows less efficiently through vessels. This thickening can raise cardiovascular risk, including higher chance of clotting events, stroke, or heart attack.
Patients on TRT should have their hematocrit and hemoglobin checked regularly through blood work. If erythrocytosis develops, a provider may recommend dose reduction, less frequent injections, switching delivery methods, or blood donation to reduce cell count. Some medical clinics specializing in TRT include hematocrit monitoring as part of standard protocol to catch this condition early and adjust treatment accordingly.