Erythrocytosis is a well-described effect of testosterone therapy in adults, but less is known about this phenomenon in transgender and gender-diverse adolescents. To explore this further, we performed a retrospective study in 158 adolescents who received gender-affirming testosterone therapy from January 2007 to April 2020 at a pediatric gender service. Erythrocytosis (hematocrit [Hct] > 0.50 L/L) developed in 9.5% (15/158) of adolescents. Frequency increased with time; in the majority (67%, 10/15), erythrocytosis occurred in the second year of treatment. Overall, our findings indicate erythrocytosis occurs commonly in testosterone-treated transgender adolescents and support the need for regular ongoing monitoring of Hct.
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