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

CONTEXT: Studies have found a variable incidence of erythrocytosis among people using testosterone as part of gender-affirming hormone therapy (GAHT). OBJECTIVE: To examine the effect of using exogenous testosterone as GAHT on hematocrit in a large North American cohort. METHODS: We conducted a cross-sectional analysis of testosterone and hematocrit laboratory values in 6670 patients who were prescribed testosterone through Plume, a national provider of GAHT. The prevalence of erythrocytosis, the mean hematocrit at predetermined testosterone thresholds and with varying routes of testosterone administration were assessed. RESULTS: Among 6670 individuals, 560 (8.4%) had a hematocrit ≥50%, 182 ≥ 52% (2.7%), and 60 ≥ 54% (0.9%). There was significant variation (P < .001) in hematocrit between different clinically relevant testosterone thresholds (T < 50 vs T 50-299 vs T 300-999 vs T ≥ 1000 ng/dL) and when comparing serum testosterone in increments of 50 ng/dL within the target range for males (300-1000 ng/dL) (P < .001). Mean hematocrit ranged from 41.84% (T < 50 ng/dL) to 45.68% (T 900-949 ng/dL). Patients on intramuscular testosterone had a higher mean hematocrit than those on transdermal testosterone (44.96% vs 43.41%, P < .001). Both route of administration (P < .001) and testosterone level (P < .001) had statistically significant associations with hematocrit when controlling for each other. CONCLUSION: While the magnitude of change in hematocrit with serum level and route of administration of testosterone was statistically significant, the absolute levels were within the normal range, unlikely to be clinically meaningful. These findings, along with the low prevalence of erythrocytosis, should help allay concerns about the use of testosterone as GAHT.