Testosterone replacement therapy (TRT) can improve symptoms and quality of life in appropriately diagnosed men, but it also has an important effect on the blood.
One of the most closely monitored parameters during TRT is haematocrit — the percentage of your blood volume made up of red blood cells.
Testosterone stimulates red blood cell production. In some men, this increase can become significant enough to cause erythrocytosis, commonly described in TRT guidelines as a haematocrit above 54%.
So what should you do if your haematocrit comes back high?
And could something as simple as dehydration or a hard gym session before your blood test make the result look worse than it really is?
Let’s look at what the evidence actually says.
What is haematocrit?
Haematocrit (Hct) is the proportion of your blood made up of red blood cells.
For example, a haematocrit of 45% means that approximately 45% of your blood volume consists of red blood cells.
Red blood cells carry oxygen around the body, so having an adequate number is essential.
The problem is that too many red blood cells can increase blood viscosity. A persistently elevated haematocrit therefore deserves medical attention, particularly when it occurs during testosterone treatment.
Importantly, a high haematocrit doesn’t automatically mean that something dangerous is happening. There are several possible explanations, including testosterone treatment itself, dehydration, smoking, sleep apnoea, lung disease and other medical conditions.
Why does TRT increase haematocrit?
Testosterone stimulates the body’s production of red blood cells.
The exact mechanism is complex, but testosterone appears to increase erythropoietin activity while also affecting hepcidin and iron metabolism, ultimately promoting erythropoiesis — the production of red blood cells.
This isn’t a rare or unexpected effect.
A systematic review and network meta-analysis involving 29 placebo-controlled randomised trials and 3,393 men found that all major testosterone formulations studied increased haematocrit compared with placebo. The average increase varied by formulation, with intramuscular testosterone enanthate/cypionate producing an approximately 4 percentage-point increase compared with placebo.
The formulation matters too.
Short-acting injectable testosterone, including testosterone cypionate and enanthate, has historically been associated with a greater tendency towards erythrocytosis than some transdermal preparations.
That doesn’t mean injections are inappropriate. It means haematocrit needs to be monitored and the treatment adjusted when necessary.
What counts as a high haematocrit on TRT?
There isn’t one universally agreed threshold for every clinical situation.
However, 54% is an important threshold in testosterone-treatment guidelines.
The British Society for Sexual Medicine recommends keeping haematocrit below 54% and recommends monitoring it before treatment, at 3–6 months, at 12 months and annually thereafter. If it rises above 54%, dose reduction or a change of preparation should be considered; if it remains elevated, stopping treatment and restarting at a lower dose may be appropriate.
The Endocrine Society similarly recommends evaluating men who develop erythrocytosis, withholding testosterone until the haematocrit returns to the normal range and then restarting at a lower dose.
So 54% isn’t simply a number to ignore, but neither should one isolated result automatically lead to panic.
The result needs to be interpreted in context.
Could dehydration make your haematocrit look high?
Yes.
This is one of the most important things to understand about a blood test.
Haematocrit is a ratio involving red blood cells and plasma. If you become dehydrated and your plasma volume falls, the concentration of red blood cells can appear higher even though your actual red-cell mass hasn’t necessarily increased.
Medical references recognise dehydration as a potential cause of an elevated haematocrit.
This is sometimes called relative erythrocytosis or haemoconcentration.
That means that if you’ve spent the previous day:
- drinking very little water
- training hard
- sweating heavily
- using a sauna
- drinking significant amounts of alcohol
- or otherwise becoming dehydrated
your blood concentration may not represent your usual hydrated state.
Exercise itself can temporarily reduce plasma volume and increase measured haematocrit. Studies of acute exercise have demonstrated haemoconcentration and changes in plasma volume following moderate-to-high intensity exercise.
How should you prepare for a haematocrit blood test?
The objective isn’t to artificially lower your haematocrit.
The objective is to make sure the result reflects your normal physiological state.
1. Stay normally hydrated
Don’t arrive at your blood test dehydrated.
Drink fluids normally in the hours and day before the test.
You don’t need to excessively drink litres of water immediately beforehand. In fact, deliberately overhydrating isn’t a sensible way to manipulate a blood result.
The goal is simply to arrive normally hydrated.
2. Avoid an unusually hard workout immediately beforehand
If possible, avoid a particularly strenuous training session immediately before your blood draw.
Hard exercise can temporarily reduce plasma volume and produce haemoconcentration, potentially increasing haematocrit temporarily.
This is particularly relevant if you regularly train hard and have your bloods taken immediately after training.
3. Don’t deliberately manipulate the result
This is important.
If your haematocrit is genuinely elevated because of your TRT dose, dehydration before the test doesn’t solve the underlying problem.
A normal blood test should represent your normal health, not your best possible laboratory result.
4. Follow your clinic’s instructions
If you’re having a broader TRT monitoring panel, there may be specific instructions concerning fasting, timing of testosterone measurements and medication.
For example, the British Society for Sexual Medicine gives formulation-specific advice for testosterone measurements and emphasises regular monitoring of haematocrit.
What if your haematocrit is still high?
This is where things become more important.
If you’ve had your blood test under normal conditions and the haematocrit remains elevated, don’t simply assume that you need to drink more water.
Your clinician should consider why it is elevated.
Potential contributors include:
Testosterone dose and formulation
Higher testosterone exposure and injectable formulations can increase the likelihood of erythrocytosis.
Sleep apnoea
Obstructive sleep apnoea can contribute to secondary erythrocytosis because repeated episodes of reduced oxygenation can stimulate red blood cell production.
This is particularly important because untreated severe obstructive sleep apnoea is also a recognised concern when considering testosterone therapy.
Smoking
Smoking is another recognised risk factor for erythrocytosis, and observational research in testosterone-treated populations has found an association between tobacco use and elevated haematocrit.
Lung disease or chronic hypoxia
Conditions that reduce oxygen availability can stimulate red blood cell production.
Other medical causes
Persistent erythrocytosis isn’t automatically caused by TRT.
If the elevation is significant or unexplained, clinicians may need to consider other causes, including primary blood disorders such as polycythaemia vera. The British Society for Haematology recommends investigating primary and secondary causes when erythrocytosis is identified.
What does the evidence say if haematocrit remains above 54%?
This is where treatment should be clinically managed rather than self-managed.
The British Society for Sexual Medicine recommends:
Reduce the testosterone dose or change the preparation.
If the haematocrit remains elevated:
Consider stopping testosterone and reintroducing it at a lower dose once appropriate.
The Endocrine Society similarly recommends withholding testosterone until the haematocrit returns to the normal range and then restarting at a lower dose.
In other words, the first question isn’t:
“How can we get the haematocrit down while keeping exactly the same treatment?”
It’s:
“Why is the haematocrit high, and can we modify the treatment or address another underlying cause?”
What about changing the testosterone formulation?
This can be an important option.
Research consistently suggests that different testosterone formulations have different effects on haematocrit.
A network meta-analysis of 29 randomised trials found that intramuscular testosterone enanthate/cypionate produced a larger average increase in haematocrit than transdermal patches, while all of the testosterone formulations studied increased haematocrit compared with placebo.
The British Society for Sexual Medicine also notes that long-acting testosterone undecanoate and transdermal gels generally have less effect on haematocrit than short-acting injectable preparations.
This doesn’t mean that changing formulation will automatically solve the problem.
It means formulation is one of the variables a doctor can consider when managing persistent erythrocytosis.
What about blood donation or venesection?
This is an area where the science is more complicated than many TRT forums suggest.
Venesection/phlebotomy can lower haematocrit, and some testosterone guidelines include it as an option for managing significant testosterone-associated erythrocytosis.
However, a 2024 review specifically examining testosterone-induced erythrocytosis highlighted an important problem:
There is limited evidence demonstrating that therapeutic phlebotomy is safe and beneficial specifically for TRT-induced erythrocytosis.
The authors point out that repeatedly removing blood can reduce iron stores and may trigger biological responses that could potentially have undesirable effects.
So venesection shouldn’t simply become:
“My haematocrit is high, so I’ll donate blood every few weeks and carry on with the same TRT dose.”
It should be a clinical decision based on the individual situation.
Does a high haematocrit definitely mean TRT is dangerous?
Not necessarily.
This is an area where we need to avoid overstating the evidence.
The Endocrine Society notes that the exact haematocrit level at which cardiovascular or neuro-occlusive risk increases isn’t known, and the frequency of such events in testosterone trials has been low.
The large TRAVERSE trial, which investigated cardiovascular safety of testosterone replacement in men with hypogonadism and cardiovascular risk, also provides useful context. A later analysis reported that only six participants developed haematocrit above 54% at the lowest testosterone dose, and within that study there was no association between changes in haematocrit and major adverse cardiovascular events or venous thromboembolism.
However, these findings shouldn’t be interpreted as evidence that persistently high haematocrit is harmless.
TRAVERSE used a specific treatment protocol and monitoring system, and the authors themselves caution that the findings may not apply to all testosterone regimens, particularly short-acting self-administered injection regimens.
That’s why clinical guidelines continue to use haematocrit monitoring and the 54% threshold when managing testosterone therapy.
What should a man on TRT actually do?
If your haematocrit is elevated, the sensible approach is:
If the result is only mildly elevated
Your doctor may repeat the test under normal conditions and consider factors such as hydration, recent exercise, smoking, sleep apnoea and your testosterone dose/formulation.
If it remains elevated
Your clinician should investigate potential causes and review your TRT regimen.
That may involve:
- reviewing your testosterone dose
- reviewing the formulation
- checking testosterone levels and timing
- assessing cardiovascular risk
- considering sleep apnoea or other causes of hypoxia
- reviewing smoking status
- investigating unexplained persistent erythrocytosis
If it reaches or exceeds 54%
This should trigger clinical action rather than simply waiting for the next routine blood test.
Current guidance supports reducing the dose or changing formulation, with temporary interruption of testosterone and/or further intervention depending on the individual circumstances.
The key takeaway
A high haematocrit on TRT doesn’t automatically mean you have to stop treatment permanently.
But it does mean something needs investigating.
First, make sure your blood test reflects your normal state:
Stay normally hydrated.
Don’t deliberately manipulate the result.
Avoid an unusually strenuous workout immediately before testing.
If the result remains high, look beyond hydration.
Your testosterone dose, formulation, sleep apnoea, smoking, lung health and other medical causes may all be relevant.
And if haematocrit reaches 54% or above, the evidence-based approach is to involve your clinician and consider modifying the testosterone regimen rather than simply trying to counteract the result with repeated blood donation.
The goal isn’t simply to make the number look better.
The goal is to find the safest effective TRT regimen for the individual.
References
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. 2018.
- Hackett G, Kirby M, et al. British Society for Sexual Medicine Guidelines on Male Adult Testosterone Deficiency, with Statements for Practice. Journal of Sexual Medicine.
- Nackeeran S, Ory J, Gonzalez D, et al. The Effect of Route of Testosterone on Changes in Hematocrit: A Systematic Review and Bayesian Network Meta-Analysis of Randomized Trials. Journal of Urology. 2022.
- Ohlander SJ, Varghese B, Pastuszak AW. Erythrocytosis Following Testosterone Therapy. Sexual Medicine Reviews.
- Bachman E, Feng R, Travison T, et al. Research into testosterone-induced erythropoiesis and haematocrit changes.
- McMullin MF, Mead AJ, Ali S, et al. A guideline for the management of specific situations in polycythaemia vera and secondary erythrocytosis. British Journal of Haematology. 2019.
- Corona G, et al./Nackeeran et al. Research examining testosterone formulations and haematocrit response.
- Hackett G, et al. Cardiovascular safety of testosterone replacement therapy (TRAVERSE).
- Testosterone therapy-induced erythrocytosis: can phlebotomy be justified? Endocrine Connections. 2024.
- Diagnosis, management, and outcomes of drug-induced erythrocytosis: a systematic review. Blood Advances. 2025.
Medical disclaimer: This article is for general information only and is not a substitute for individual medical advice. If you are receiving testosterone therapy and have an elevated haematocrit, speak to your prescribing clinician before changing, stopping or adjusting your treatment. Never alter your testosterone dose or arrange venesection/phlebotomy without appropriate medical advice.
