Summary

Testosterone makes the body produce more red blood cells. Blood that becomes too thick raises the risk of clots, heart attack and stroke. Amsterdam doctors reviewed blood tests from 1,073 trans men on testosterone over up to 20 years. About one in nine had repeatedly high levels by male standards. The chance of ever crossing the limit kept rising for years, reaching 38% after a decade, so someone stable for years can still develop it. Long-acting injections, smoking, higher weight, older age and lung disease raised the risk. The authors advise checking blood for as long as testosterone is used. More than half of the clinic's trans men were left out for missing data. Using the normal limits for women would label far more people as affected (26%, the authors said in a reply to a letter), so the true scale may be higher.

Key findings

• Z 1 073 trans mužů na testosteronu mělo 11 % příliš hustou krev (více než polovina objemu krve tvořená červenými krvinkami) ve dvou testech podle mužských standardů; 3,7 % a 0,5 % překročilo vyšší limity.
• 24 % mělo alespoň jeden krevní test nad mužským limitem.
• V průměru se podíl krve tvořený červenými krvinkami zvýšil z 39 % před léčbou na 45 % po jednom roce a poté se za 20 let zvýšil jen mírně.
• Mezi 776 muži s počátečními daty dosáhla šance, že někdy překročí mužský limit, 38 % po 10 letech a 50 % po 14 letech.
• Dlouhodobě působící injekce testosteronu souvisely s asi 3krát vyšší šancí na příliš hustou krev ve srovnání s gelem; riziko zvyšovalo také kouření (asi dvojnásobná šance), vyšší váha, vyšší věk při zahájení a plicní onemocnění.
• Při použití normálního limitu pro ženy by se ve dvou testech kvalifikovalo 26 %, řekli autoři v odpovědi na zveřejněný dopis.
• Studie měřila jen krevní testy, ne skutečné zdravotní události, například krevní sraženiny nebo infarkty.

Abstract

CONTEXT: Erythrocytosis is a known side effect of testosterone therapy that can increase the risk of thromboembolic events. OBJECTIVES: To study the prevalence and determinants in the development of erythrocytosis in trans men using testosterone. METHODS: A 20-year follow-up study in adult trans men who started testosterone therapy and had monitoring of hematocrit at our center (n = 1073). RESULTS: Erythrocytosis occurred in 11% (hematocrit > 0.50 L/L), 3.7% (hematocrit > 0.52 L/L), and 0.5% (hematocrit > 0.54 L/L) of trans men. Tobacco use (odds ratio [OR] 2.2; 95% CI, 1.6-3.3), long-acting undecanoate injections (OR 2.9; 95% CI, 1.7-5.0), age at initiation of hormone therapy (OR 5.9; 95% CI, 2.8-12.3), body mass index (BMI) (OR 3.7; 95% CI, 2.2-6.2), and pulmonary conditions associated with erythrocytosis and polycythemia vera (OR 2.5; 95% CI, 1.4-4.4) were associated with hematocrit > 0.50 L/L. In the first year of testosterone therapy hematocrit increased most: 0.39 L/L at baseline to 0.45 L/L after 1 year. Although there was only a slight continuation of this increase in the following 20 years, the probability of developing erythrocytosis still increased (10% after 1 year, 38% after 10 years). CONCLUSION: Erythrocytosis occurs in trans men using testosterone. The largest increase in hematocrit was seen in the first year, but also after the first years a substantial number of people present with hematocrit > 0.50 L/L. A reasonable first step in the care for trans men with erythrocytosis while on testosterone is to advise them to quit smoking, to switch to a transdermal administration route, and if BMI is high, to lose weight.

Conclusion

Erythrocytosis occurs in trans men using testosterone. The largest increase in hematocrit was seen in the first year, but also after the first years a substantial number of people present with hematocrit > 0.50 L/L. A reasonable first step in the care for trans men with erythrocytosis while on testosterone is to advise them to quit smoking, to switch to a transdermal administration route, and if BMI is high, to lose weight. The authors add that regular hematocrit control seems warranted for as long as testosterone is used.