In men, when their natural testosterone levels go up, their blood tends to carry more red blood cells, making their blood thicker — like going from 42% to 46% red blood cells as testosterone rises.

When people take hormone therapy to align their body with their gender, both how much testosterone is in their blood and how they get it (like shots or gel) can change their red blood cell count — and each one matters on its own.

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.

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.

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.

If your testosterone levels go as high as they possibly can without being considered abnormal, it might cause serious side effects in some people.