Testosterone causes red blood cells to become larger and more numerous. If you are a trans man or transmasculine person on testosterone, your red blood cell profile will shift to look like that of other men. That is not a risk in itself; it is simply testosterone doing what testosterone does in any body it runs through.
What does testosterone actually do to red blood cells?
Testosterone stimulates the production of erythropoietin, a hormone made in the kidneys that tells bone marrow to make more red blood cells. The result is predictable and consistent: red blood cells become slightly larger, slightly denser, and there are more of them. Your haematocrit (the proportion of your blood that is red blood cells) and haemoglobin (the protein inside them that carries oxygen) both rise. This is true for any person on testosterone, trans or not.
Before testosterone, if your hormone profile was oestrogen-dominant, your red blood cell values sat in the reference range typical for people with that profile. Once testosterone takes over, those values migrate toward the male reference range. That migration is the point. It is testosterone working correctly.
Is a higher red blood cell count dangerous?
Not inherently, no. Every man walking around has the same profile you are moving toward. The values that look elevated against a female reference range are entirely normal against a male one. This is why it matters which reference range your lab result is being read against: a haemoglobin of 16 g/dL looks alarming on a form printed with female norms and entirely unremarkable on one printed with male norms.
There is one genuine thing to monitor, and that is polycythaemia: a condition where red blood cell production runs too high, making the blood thicker and raising the small risk of clotting. This can happen if testosterone doses run significantly above the therapeutic range for longer periods. It is not a common outcome of well-managed care; it is a reason to have regular blood tests so that your prescriber can see the numbers and adjust if needed. Monitoring is the tool here, not anxiety.
If your haematocrit is creeping above around 52 percent, that is the conversation to have with whoever manages your prescription. Below that, the values are simply telling you that testosterone is doing its job.
How does this compare to cisgender men?
Identically. A cisgender man with the same haemoglobin and haematocrit as yours would not be told his blood is a risk factor. He would be told his bloods look fine. The only reason this comes up in trans healthcare at all is that some labs still report results against a single sex-based reference range without adjusting for the person's hormone profile, and some clinicians are more familiar with that framing than with trans-specific interpretation.
Good gender-affirming care, as described in the WPATH Standards of Care 8 and the Endocrine Society clinical practice guidelines, includes regular haematological monitoring specifically so that prescribers can confirm values are landing in a healthy male range rather than running above it. The monitoring is routine, not a red flag.
What should my blood tests actually be measuring?
The key values in routine testosterone monitoring are haemoglobin, haematocrit, and sometimes a full blood count. Your prescriber should be reading those results against male reference ranges, because that is the range appropriate to your hormone profile. If a result comes back flagged as high and you are not sure which range was used, it is worth asking. A flag generated by a female reference range applied to a person on testosterone is not a clinical finding; it is a labelling mismatch.
Does this mean I need to drink more water or change my diet?
Staying well hydrated is sensible general health advice for everyone, and it is true that good hydration keeps blood viscosity in a comfortable range. But I would not frame this as a special burden of being on testosterone. Cisgender men do not receive lectures about hydration as a side effect of their male biology. Drink water because it is good for you, not because testosterone has made you medically precarious.
There is no specific dietary intervention required or recommended because of this change in your blood profile. Eating well supports your health generally; it is not a corrective measure for something gone wrong.
What if my GP seems concerned about my levels?
It is worth having a direct conversation about which reference range their concern is based on. GPs who see very few trans patients sometimes apply female reference ranges out of habit, or because their laboratory software defaults to that. A haemoglobin that looks high on paper may be exactly where it should be for a man on testosterone.
If you are in a position where your GP is genuinely uncertain about how to interpret your results in the context of testosterone therapy, a letter or report from whoever manages your prescription, whether that is a private gender clinic or a specialist, can be a useful bridge. You should not have to teach your GP everything from scratch, but sometimes a single clear explanation makes the next ten appointments easier.
The short version
Testosterone makes red blood cells larger, denser, and more numerous. That is normal male physiology. Your blood is not doing something unusual; it is doing what it is supposed to do. Regular monitoring makes sure the numbers land in the right range rather than above it, and adjustments to your prescription are straightforward if they are ever needed. There is nothing here to be alarmed about, and quite a lot to feel settled by.
If there is a topic that you would like me to cover, just let Sammy know.
Words by Dr Helen Webberley, Gender Specialist and Medical Educator, founder of GenderGP, and advocate for trans rights and gender diversity worldwide.
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