Testosterone tells your bone marrow to make more red blood cells. That is not a malfunction, it is the hormone doing what it does. Taken too far it thickens the blood, and thick blood is a genuine problem. It is the most common measurable effect of treatment and the easiest one to catch, provided somebody is actually looking.
Ask most men what worries them about testosterone treatment and they will say the prostate or the heart. Ask a specialist and the answer is your hematocrit.
What hematocrit is
It is the proportion of your blood made up of red cells, expressed as a percentage. It appears on any routine full blood count, which means it is cheap, universally available, and completely unmissable if anybody bothers to look.
Testosterone raises it. It stimulates erythropoietin and it makes iron more available to the marrow, and the marrow does what it is told. In a man who was genuinely deficient, a rise from a low starting point is part of the benefit. Roy and colleagues found that low testosterone in older men was associated with anemia, and that treatment corrected it in a substantial share of those men.
The same mechanism that fixes an anemia is the one that causes the problem when it overshoots.
Why thick blood matters
Blood that carries a higher proportion of cells flows less easily. The theoretical concern is clotting, and while the size of that risk in treated men is debated, nobody argues that a steadily climbing hematocrit should be ignored.
It is also the single clearest sign that a dose is too high, or that the way it is being delivered is producing peaks that are too sharp. In that sense it is useful information rather than only a hazard.
The number that triggers action
The Endocrine Society guideline sets the threshold at a hematocrit above 54 percent. Above that, treatment should be stopped or the dose reduced, and the cause investigated, until it returns to a safe range.
Below that, a rise is watched rather than acted on. A man who starts low and moves into the normal range has not developed a problem, he has stopped being anemic.
What is done about it
A clinical guide published in European Urology Focus sets out the options, and the least dramatic ones come first. It is a finding that is usually managed rather than a reason to abandon anything, provided somebody is reading it.
Other contributors get looked for at the same time. Smoking raises it. Untreated sleep apnea raises it. Dehydration raises it artificially and produces a frightening number that resolves with a glass of water and a repeat test.
Which is the point: a single high reading is not a diagnosis. It is a reason to repeat it properly and to look at why.
When it tends to happen
It is not immediate. The rise builds over the first months, which is why the schedule of checks is front loaded rather than annual, and why a man who had one blood test at the start and nothing since has been monitored in name only.
It is also more pronounced in older men, in smokers, and in men whose delivery method produces high peaks rather than steady levels. None of those are reasons not to treat. All of them are reasons to know which category you are in before you start.
Why this is the argument for supervision
This effect is dose related, it is silent, and it is trivially detectable. A man on treatment without a scheduled blood count is not being monitored, whatever he has been told.
It is also the reason that treatment obtained without a doctor is not the same product as treatment given by one. The molecule may be identical. The blood count is the part that is missing.
What else is checked and when is covered in is testosterone therapy safe, honestly. Why the assessment beforehand decides everything is in who should actually assess you.
Agrawal P, et al. "Management of Erythrocytosis in Men Receiving Testosterone Therapy: Clinical Consultation Guide." European Urology Focus, 2023. doi:10.1016/j.euf.2022.10.008
Roy CN, et al. "Association of Testosterone Levels With Anemia in Older Men." JAMA Internal Medicine, 2017. doi:10.1001/jamainternmed.2016.9540
Bhasin S, et al. "Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline." The Journal of Clinical Endocrinology and Metabolism, 2018. doi:10.1210/jc.2018-00229