By Nelson Vergel | ExcelMale.com | Last updated: May 2026
If you've been on testosterone replacement therapy long enough, you've probably heard the word "hematocrit" more than you'd like. Your doctor orders labs, the number creeps up, and at some point someone mentions therapeutic phlebotomy — a blood draw to bring things back down. It works. It's also annoying, requires repeat visits, and leaves some men feeling wiped out for days afterward.
A drug called rusfertide is now in the process of seeking regulatory approval for polycythemia vera, a blood cancer that causes runaway red blood cell production. Phase III trial data presented at the 2025 ASCO Annual Meeting showed it more than doubled clinical response rates and dramatically reduced the need for phlebotomy. When I read the mechanism, something clicked: the biology driving elevated hematocrit in polycythemia vera and the biology driving it in TRT patients isn't the same thing — but it runs through the same chokepoint. That's worth unpacking.
On TRT, both mechanisms operate. More EPO drives the bone marrow harder. Lower hepcidin makes sure there's enough iron to sustain that production. The result: hematocrit climbs, blood viscosity increases, and cardiovascular risk edges up — particularly above 54% in most clinical guidelines.
This is dose-dependent. Higher testosterone doses produce more erythrocytosis. Injectable testosterone, especially long-acting formulations, tends to cause more of it than topical gels or creams, partly due to peak-level effects.
Rusfertide is a synthetic peptide that mimics hepcidin. It binds to ferroportin — the transmembrane protein that exports iron out of cells into the bloodstream — and triggers its degradation. Less ferroportin means less iron reaching the bone marrow. Less iron in the marrow means fewer red blood cells, and hematocrit comes down.
In the VERIFY phase III trial, rusfertide cut phlebotomy rates from 1.8 procedures per patient down to 0.5 over 32 weeks. And 62.6% of patients maintained hematocrit below 45% compared to 14.4% on placebo.
In TRT erythrocytosis, the cause is different (exogenous testosterone suppressing hepcidin, not a JAK2 mutation), but the downstream mechanism is shared. Rusfertide, at least on paper, would cut off iron supply to the marrow regardless of why hepcidin is low.
The mechanistic argument is real. Several researchers have proposed hepcidin-based interventions as a potential way to manage TRT-induced erythrocytosis without phlebotomy. The logic is the same: block iron delivery, blunt RBC overproduction, control hematocrit. One could argue TRT patients are actually a cleaner model than PV patients — the cause is exogenous and dose-adjustable, rather than clonal and autonomous.
But there are meaningful differences that complicate the picture:
The iron status difference is worth flagging. In PV patients, iron deficiency coexists with excess RBC production — rusfertide's mechanism makes particular sense there because restricting iron to the marrow is therapeutic, not harmful. TRT patients with normal iron stores may have a different risk profile if iron restriction goes too far.
There's no dosing data, titration protocol, or safety profile established for TRT-induced erythrocytosis. What dose would you use? How low can hematocrit safely go? What happens when someone stops TRT while on rusfertide? None of that is known.
This isn't a reason to dismiss the concept. It's a reason to wait for actual trials before drawing clinical conclusions.
The trajectory is worth watching. If rusfertide earns approval for PV and accumulates a real-world safety record, researchers may begin investigating erythrocytosis in other contexts — including testosterone therapy. There's already published interest in hepcidin-based approaches to managing iatrogenic polycythemia. This drug gives that idea a concrete candidate.
• Dose reduction. The most direct intervention. Lowering your testosterone dose or total weekly exposure reliably reduces erythrocytosis in most men.
• Injection frequency adjustment. Splitting the same weekly dose into more frequent smaller injections reduces peak testosterone levels and often reduces hematocrit response.
• Therapeutic phlebotomy. Donation (if eligible) or therapeutic draw keeps hematocrit in range. It doesn't fix the underlying cause but it works.
• Formulation switch. Some men see lower hematocrit on gels or creams compared to injectables, due to lower peak levels. This varies individually.
• Rule out sleep apnea. Untreated OSA independently raises EPO and hematocrit. Some men who address sleep apnea see significant improvement without any TRT changes.
• Stay hydrated. Dehydration concentrates blood and produces spuriously high hematocrit readings. Consistent hydration doesn't prevent true erythrocytosis but avoids false alarms.
What's not indicated is tolerating persistently elevated hematocrit (above 52–54%) without intervention.
What it isn't — at least not yet — is a clinical option for TRT patients. No trials, no dosing guidance, a real anemia risk, and a regulatory timeline that probably stretches years into the future. If you're managing high hematocrit on testosterone today, dose optimization and phlebotomy remain your tools.
But this is worth watching. It's one of the cleaner examples of PV research potentially unlocking something useful for the broader testosterone therapy space. When trial data in non-PV erythrocytosis start appearing — and eventually they will — ExcelMale will cover it.
In the meantime, join the discussion in our Testosterone Side Effect Management forum and share your experience managing hematocrit on TRT.
What you'll learn in this article: • Why testosterone raises hematocrit and what role hepcidin plays • What rusfertide is and how it controls hematocrit in polycythemia vera • Why the biology of TRT-induced erythrocytosis overlaps with PV — and what that could mean • Why rusfertide is not a current option for TRT men (and what the real risks are) • What to do now if high hematocrit is a problem on your protocol |
If you've been on testosterone replacement therapy long enough, you've probably heard the word "hematocrit" more than you'd like. Your doctor orders labs, the number creeps up, and at some point someone mentions therapeutic phlebotomy — a blood draw to bring things back down. It works. It's also annoying, requires repeat visits, and leaves some men feeling wiped out for days afterward.
A drug called rusfertide is now in the process of seeking regulatory approval for polycythemia vera, a blood cancer that causes runaway red blood cell production. Phase III trial data presented at the 2025 ASCO Annual Meeting showed it more than doubled clinical response rates and dramatically reduced the need for phlebotomy. When I read the mechanism, something clicked: the biology driving elevated hematocrit in polycythemia vera and the biology driving it in TRT patients isn't the same thing — but it runs through the same chokepoint. That's worth unpacking.
Why Does TRT Raise Hematocrit?
Testosterone stimulates erythropoiesis through at least two pathways. It increases erythropoietin (EPO) production in the kidneys, which tells bone marrow to make more red blood cells. It also suppresses hepcidin, a liver-derived hormone that controls how much iron reaches the bone marrow. When hepcidin is low, iron flows freely to marrow erythroid precursors, fueling RBC production. When it's high, iron is locked away and RBC output slows.On TRT, both mechanisms operate. More EPO drives the bone marrow harder. Lower hepcidin makes sure there's enough iron to sustain that production. The result: hematocrit climbs, blood viscosity increases, and cardiovascular risk edges up — particularly above 54% in most clinical guidelines.
This is dose-dependent. Higher testosterone doses produce more erythrocytosis. Injectable testosterone, especially long-acting formulations, tends to cause more of it than topical gels or creams, partly due to peak-level effects.
What Rusfertide Does — and Why It's Relevant Here
In polycythemia vera, a JAK2 gene mutation drives autonomous red blood cell overproduction. Hepcidin is also chronically suppressed in PV, which means iron flows to the marrow regardless of what the body needs. Patients are simultaneously producing too many red cells and are iron-deficient in peripheral tissues.Rusfertide is a synthetic peptide that mimics hepcidin. It binds to ferroportin — the transmembrane protein that exports iron out of cells into the bloodstream — and triggers its degradation. Less ferroportin means less iron reaching the bone marrow. Less iron in the marrow means fewer red blood cells, and hematocrit comes down.
In the VERIFY phase III trial, rusfertide cut phlebotomy rates from 1.8 procedures per patient down to 0.5 over 32 weeks. And 62.6% of patients maintained hematocrit below 45% compared to 14.4% on placebo.
The chokepoint rusfertide targets — hepcidin suppression leading to unrestricted iron delivery to bone marrow — is the same one activated by testosterone. That's the mechanistic overlap worth paying attention to. |
In TRT erythrocytosis, the cause is different (exogenous testosterone suppressing hepcidin, not a JAK2 mutation), but the downstream mechanism is shared. Rusfertide, at least on paper, would cut off iron supply to the marrow regardless of why hepcidin is low.
How Strong Is the Case for Off-Label Use in TRT Patients?
Biologically plausible. Clinically untested. Those two things matter equally here.The mechanistic argument is real. Several researchers have proposed hepcidin-based interventions as a potential way to manage TRT-induced erythrocytosis without phlebotomy. The logic is the same: block iron delivery, blunt RBC overproduction, control hematocrit. One could argue TRT patients are actually a cleaner model than PV patients — the cause is exogenous and dose-adjustable, rather than clonal and autonomous.
But there are meaningful differences that complicate the picture:
Factor | Polycythemia Vera | TRT Erythrocytosis |
Root cause | JAK2 mutation; autonomous overproduction | Testosterone-driven EPO stimulation + hepcidin suppression |
Iron status | Systemic iron deficiency is common | Often normal; iron deficiency not typical |
Reversibility | Chronic; not reversible without treatment | Dose-adjustable; reversible if TRT is stopped |
Symptom burden | Fatigue, itching, night sweats, bone pain | Often asymptomatic unless hematocrit is very high |
Standard management | Phlebotomy, hydroxyurea, ruxolitinib | Dose reduction, injection frequency adjustment, phlebotomy |
The iron status difference is worth flagging. In PV patients, iron deficiency coexists with excess RBC production — rusfertide's mechanism makes particular sense there because restricting iron to the marrow is therapeutic, not harmful. TRT patients with normal iron stores may have a different risk profile if iron restriction goes too far.
What About Safety? The Anemia Risk Is Real
In the VERIFY trial, 15.9% of patients in the rusfertide arm developed anemia. In PV, controlled hematocrit reduction is the goal, so anemia is a manageable tradeoff. In a TRT patient whose primary goal is optimal energy and wellbeing — not cancer control — tipping into anemia would be a meaningful downside.There's no dosing data, titration protocol, or safety profile established for TRT-induced erythrocytosis. What dose would you use? How low can hematocrit safely go? What happens when someone stops TRT while on rusfertide? None of that is known.
This isn't a reason to dismiss the concept. It's a reason to wait for actual trials before drawing clinical conclusions.
Where Does This Leave Us? The Regulatory Reality
Rusfertide is not yet approved anywhere. The VERIFY data were submitted to regulators to support a marketing authorization filing for PV, not TRT. Even in the most optimistic scenario, off-label use in TRT patients is at minimum several years away from any rational clinical framework.The trajectory is worth watching. If rusfertide earns approval for PV and accumulates a real-world safety record, researchers may begin investigating erythrocytosis in other contexts — including testosterone therapy. There's already published interest in hepcidin-based approaches to managing iatrogenic polycythemia. This drug gives that idea a concrete candidate.
What Should Men on TRT Do About High Hematocrit Right Now?
The current toolkit is effective, even if imperfect. If your hematocrit is running high on TRT, here's what's supported by evidence:• Dose reduction. The most direct intervention. Lowering your testosterone dose or total weekly exposure reliably reduces erythrocytosis in most men.
• Injection frequency adjustment. Splitting the same weekly dose into more frequent smaller injections reduces peak testosterone levels and often reduces hematocrit response.
• Therapeutic phlebotomy. Donation (if eligible) or therapeutic draw keeps hematocrit in range. It doesn't fix the underlying cause but it works.
• Formulation switch. Some men see lower hematocrit on gels or creams compared to injectables, due to lower peak levels. This varies individually.
• Rule out sleep apnea. Untreated OSA independently raises EPO and hematocrit. Some men who address sleep apnea see significant improvement without any TRT changes.
• Stay hydrated. Dehydration concentrates blood and produces spuriously high hematocrit readings. Consistent hydration doesn't prevent true erythrocytosis but avoids false alarms.
What's not indicated is tolerating persistently elevated hematocrit (above 52–54%) without intervention.
Frequently Asked Questions
Is rusfertide available for TRT patients right now?
No. As of mid-2026, rusfertide has no regulatory approval anywhere, including for polycythemia vera. Clinical trials in TRT-induced erythrocytosis have not been conducted. It is not accessible as a treatment option for this purpose.Does testosterone directly suppress hepcidin?
Yes. Testosterone suppresses hepcidin through EPO-mediated pathways and possibly through direct hepatic effects. This is well-established and is one of the primary reasons TRT increases hematocrit. The hepcidin/ferroportin axis is a real target in this context — rusfertide just happens to be the most advanced drug acting on it.Is high hematocrit on TRT dangerous?
It can be. Hematocrit above 54% is associated with increased blood viscosity and cardiovascular risk, including stroke and deep vein thrombosis. Most guidelines recommend intervention — whether dose adjustment or phlebotomy — when hematocrit exceeds 52–54%. The absolute risk in any individual depends on other factors including blood pressure, smoking status, and platelet function.Are there other drugs being studied for TRT-induced erythrocytosis?
There is published interest in hepcidin analogs and inducers as a class. Minihepcidins — shorter synthetic hepcidin peptides — have been studied in preclinical models. No drug has reached clinical trials specifically targeting TRT-induced erythrocytosis, but this is an active area in iron biology research.What hematocrit level should prompt me to call my doctor?
Most TRT guidelines suggest discussing intervention when hematocrit exceeds 52–54%. If you're at 50–52%, it's worth flagging at your next appointment, especially if it's trending up. Don't wait for symptoms — significantly thickened blood often doesn't produce obvious ones until something goes wrong.The Bottom Line
Rusfertide is a legitimately interesting drug for anyone following TRT side effect management. The hepcidin/ferroportin pathway it targets is the same one testosterone disrupts, and the mechanistic case for applying it to TRT erythrocytosis is coherent. The VERIFY phase III data in PV are strong enough that regulatory approval looks likely.What it isn't — at least not yet — is a clinical option for TRT patients. No trials, no dosing guidance, a real anemia risk, and a regulatory timeline that probably stretches years into the future. If you're managing high hematocrit on testosterone today, dose optimization and phlebotomy remain your tools.
But this is worth watching. It's one of the cleaner examples of PV research potentially unlocking something useful for the broader testosterone therapy space. When trial data in non-PV erythrocytosis start appearing — and eventually they will — ExcelMale will cover it.
In the meantime, join the discussion in our Testosterone Side Effect Management forum and share your experience managing hematocrit on TRT.
Medical Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting or modifying any hormone therapy or medical treatment. Rusfertide is not approved for use in TRT-induced erythrocytosis and should not be sought for this purpose outside of a clinical trial context. |
