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Supported

Giving someone testosterone can make their body produce more red blood cells, which thickens the blood; if the blood gets too thick, it might increase the chance of dangerous blood clots.

53
Pro
35
Against

Evidence from Studies

Supporting (2)

53

Community contributions welcome

48

Adverse events associated with testosterone replacement in middle-aged and older men: a meta-analysis of randomized, placebo-controlled trials.

Systematic Review With Meta-Analysis
Human
2005 Nov

This study found that when older men take testosterone, their blood gets thicker (higher hematocrit), which can make blood clots more likely. That’s exactly what the claim says.

13

Hepcidin is not essential for mediating testosterone's effects on erythropoiesis

Randomized Controlled Trial
Animal
2020 Jan

This study found that giving testosterone to mice makes their bodies produce more red blood cells and increases the percentage of red blood cells in their blood — exactly what the claim says. It doesn’t prove the blood gets too thick and causes clots, but it does prove the first big step is real.

Contradicting (2)

35

Community contributions welcome

35

Erythrocytosis Is Rare With Exogenous Testosterone in Gender-Affirming Hormone Therapy.

Cross-Sectional Study
Human
2024 Apr 19

This study found that even when people take testosterone, their blood usually doesn’t get thick enough to be dangerous—only a tiny fraction reached levels that might raise clot risk, so it doesn’t support the idea that testosterone commonly causes dangerous blood thickening.

1

Testosterone deficiency and treatments: common misconceptions and practical guidance for patient care.

Narrative Review
Human
2025 Jul 3

This study says testosterone therapy doesn’t increase heart risks, but it never looked at whether it raises red blood cell count or causes blood clots — so it doesn’t prove or disprove the specific claim about hematocrit and thrombosis.

Gold Standard Evidence Needed

According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.

Similar Assertions

Giving testosterone to female mice makes their blood carry more oxygen by increasing certain blood cells, even when a key regulatory protein called hepcidin is missing or when the liver can't respond to testosterone—meaning testosterone might boost blood cell production on its own.

13
0
80%

For men with low testosterone, taking testosterone therapy might slightly raise the number of red blood cells, but it doesn’t seem to make blood clots or lung blockages any more likely over almost two years of use.

1
0
78%

When people take testosterone as part of gender-affirming care, their blood can get a bit thicker from more red blood cells, but even at its highest, it’s still in a safe, normal range — so serious thickening of the blood is uncommon.

35
0
74%

For men over 45 taking testosterone therapy, their blood often gets thicker with more red cells, making it almost four times more likely to reach unsafe levels—so doctors need to check their blood regularly.

48
0
73%

For men with low testosterone, giving them testosterone therapy can help fix their low red blood cell count and make them less anemic, without raising their risk of heart problems.

1
0
73%

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