Yes, testosterone directly increases red blood cell production. This is one of the most common side effects of testosterone therapy, with hematocrit (the percentage of your blood made up of red blood cells) rising 7% to 10% above baseline in people taking testosterone. The effect is driven by two biological mechanisms that work together to push your body into making more red blood cells than it normally would.
How Testosterone Increases Red Blood Cells
Testosterone raises red blood cell counts through a two-pronged process. First, it stimulates your kidneys to produce more erythropoietin (EPO), the hormone that tells your bone marrow to make red blood cells. In one study published in The Journals of Gerontology, EPO levels jumped 58% within the first month of testosterone administration. But testosterone doesn’t just cause a temporary spike. It actually recalibrates the relationship between EPO and hemoglobin, shifting the body’s internal thermostat so that EPO levels run about 30% higher than they otherwise would at any given hemoglobin level. In other words, your body resets what it considers “normal” and starts targeting a higher red blood cell count.
Second, testosterone suppresses hepcidin, a hormone that controls how much iron is available in your bloodstream. With hepcidin levels dropping by nearly 50%, more iron gets released from your body’s storage sites and funneled into red blood cell production. Ferritin (stored iron) drops while signs of active iron use increase. This combination of more EPO signaling and more available iron creates the perfect conditions for your bone marrow to ramp up output.
How Common Is This Effect?
The likelihood of developing an elevated red blood cell count depends heavily on how you take testosterone. In a study comparing delivery methods, 66.7% of people using injectable testosterone developed a hematocrit above 50%, compared to 35.1% of those using pellets and 12.8% of those using topical gels. Injections create higher peak levels of testosterone in the blood, which drives a stronger signal to produce red blood cells.
A large long-term study of over 1,000 transgender men using testosterone tracked hematocrit levels across 20 years. About 11% developed persistent erythrocytosis, defined as repeatedly having a hematocrit above 50%. A single elevated reading was even more common, occurring in 24% of participants at some point. More severe elevations above 54% were rare, showing up persistently in only 0.5% of the group.
Factors That Raise Your Risk
Not everyone on testosterone will see the same degree of red blood cell increase. Several factors can amplify the effect. The method of administration matters most, with injections carrying the highest risk. Beyond that, the dose itself plays a role, since testosterone has a dose-dependent effect on red blood cell production.
Smoking appears to compound the problem. While the statistical association is modest, smokers on testosterone trend toward higher hematocrit levels than nonsmokers. Untreated obstructive sleep apnea, which causes repeated drops in blood oxygen overnight, can also push red blood cell counts higher independently of testosterone. When both are present, the effects may stack. Age is another consideration: research has shown more pronounced hematocrit increases in men over 60, though this finding hasn’t been consistent across all studies. Certain diabetes medications (SGLT-2 inhibitors) have also been flagged as potential contributors.
Why It Matters for Your Health
A modest increase in red blood cells isn’t inherently dangerous, but a significant rise makes your blood thicker and harder to pump. This increased viscosity raises the risk of blood clots, including deep vein thrombosis (clots in the legs) and pulmonary embolism (clots in the lungs). Case reports have documented life-threatening clotting events in people on testosterone who had no other risk factors for blood clots, with the elevated hematocrit identified as the primary cause.
The threshold that most medical organizations consider dangerous is a hematocrit of 54%. At that level, the risk of venous thromboembolism and cardiovascular events rises enough that intervention is recommended. The American Urological Association’s guidelines suggest that if hematocrit exceeds 50%, the cause should be investigated, and at 54% or above, testosterone therapy should be reduced or paused.
Monitoring and Blood Work
Anyone starting testosterone therapy should have a complete blood count drawn before treatment begins to establish a baseline. Regular blood work continues throughout treatment. The most important number to watch is hematocrit. If it climbs above 54%, the standard approach is to lower the testosterone dose, temporarily stop therapy, or both, then recheck levels. If testosterone is restarted after hematocrit normalizes, it’s typically resumed at a lower dose with closer monitoring going forward.
Switching from injections to a topical gel is another strategy, since gels produce more stable testosterone levels without the sharp peaks that drive red blood cell production most aggressively.
Managing High Red Blood Cell Counts on Testosterone
The first-line approach to testosterone-induced erythrocytosis is adjusting the dose or changing the delivery method. Addressing contributing factors like smoking or untreated sleep apnea can also help bring levels down. These steps should be tried before anything more invasive.
Therapeutic phlebotomy, essentially having about 500 mL of blood drawn at regular intervals, is commonly mentioned in clinical guidelines as a backup option. Multiple endocrine societies reference it as a way to reduce hematocrit, with the goal of bringing levels below 50% and keeping them there. Some organizations suggest it can be done alongside continued testosterone use rather than requiring you to stop therapy entirely. The European Association of Urology, for instance, notes that one or two phlebotomy sessions combined with a dose reduction may be sufficient, without needing to discontinue testosterone altogether.
That said, the evidence supporting phlebotomy’s long-term effectiveness in this specific situation is limited. A 2024 review in Endocrine Connections noted a lack of rigorous data on whether phlebotomy actually reduces clotting risk in testosterone users, as opposed to simply lowering a number on a lab report. Repeated phlebotomy also depletes iron stores over time, which can cause its own problems. The current expert consensus is to try dose adjustment and lifestyle changes first, then discuss phlebotomy as a shared decision if those measures aren’t enough.