Erythrocytosis refers to thickening of the blood due to increased red blood cell production. The thickness of the blood is measured with a blood test for haematocrit, which is an estimated percentage by volume of red blood cells in the blood.
Testosterone increases the production of red blood cells by the body, which can increase the thickness of the blood. This accounts for why cis men tend to have a higher haematocrit than cis women. Accordingly, exogenous testosterone in masculinising hormone treatment can be associated with increased haematocrit.
In the majority of cases, masculinising hormone treatment in trans men increases the haematocrit to levels usually considered healthy in cis men.
Sometimes, however, masculinising hormone treatment in trans men can thicken the blood further, so that the haematocrit rises above the levels usually considered healthy in cis men. A recent study suggested that polycythaemia can affect around 11% of trans men (Madsen et al., 2021). The risk of erythrocytosis appears to be highest in the first year of treatment and also appears to be higher with higher doses of testosterone.
Erythrocytosis is a potential concern because it may be associated with an increased risk of blood clots, heart attacks, and strokes (Kerrebrouck et al., 2022; Olivas-Martinez et al., 2021).
For this reason, it is recommended that people who are taking testosterone have blood tests to check their haematocrit regularly.
If your haematocrit is raised above 53%, then this indicates erythrocytosis.
In this case, it is recommended that you discontinue testosterone temporarily. This is to prevent further elevation of haematocrit, which could increase the risk of a blood clot or stroke.
It is advised that you remain well-hydrated, as dehydration can contribute to raised haematocrit.
It is advised that you consult your doctor, who may investigate other potential causes of erythrocytosis. These include smoking, being overweight, obstructive sleep apnoea, and chronic lung conditions. Hence, the overall risk of erythrocytosis can be reduced by stopping smoking, maintaining a healthy body weight, and optimising treatment for any relevant medical conditions.
After undertaking the above, it is advised that you have a repeat blood test in three months to recheck your haematocrit. If your haematocrit has decreased below 53%, then it is safe to recommence testosterone.
After recommencing testosterone, it is recommended that you continue regular checking of your haematocrit to monitor for any recurrence of erythrocytosis.
Kerrebrouck, M., Vantilborgh, A., Collet, S., and T’Sjoen, G. (2022). "Thrombophilia and Hormonal Therapy in Transgender Persons: A Literature Review and Case Series". International Journal of Transgender Health, 23: 377–391.
Madsen, M. C., van Dijk, D., Wiepjes, C. M., Conemans, E. B., Thijs, A. and den Heijer, M., (2021). "Erythrocytosis in a Large Cohort of Trans Men Using Testosterone: A Long-Term Follow-Up Study on Prevalence, Determinants, and Exposure Years". Journal of Clinical Endocrinology and Metabolism, 106: 1710–1717.
Olivas-Martinez, A., Corona-Rodarte, E., Nuñez-Zuno, A., Barrales-Benítez, O., Oca, D. M. D., Mora, J. D. L., León-Aguilar, D., Hernández-Juárez, H. E., and Tuna-Aguilar, E. (2021) "Causes of Erythrocytosis and Its Impact As a Risk Factor for Thrombosis According to Etiology: Experience in a Referral Center in Mexico City". Blood Rresearch, 56: 166–174.