Abstract
PURPOSE: This study aimed to explore the use of serum free testosterone concentrations in transmasculine patients as a more reliable indicator of gender-affirming testosterone therapy than total testosterone concentration. Total testosterone values, recommended by current guidelines, may be impacted by obesity. In addition, this study aimed to characterize the impact of testosterone concentrations on hematocrit and the risk of polycythemia. METHODS: A retrospective study was conducted of transmasculine patients seen at Boston Children's Hospital Gender Multispecialty Service from 2007 to 2020. Sixty-eight birth-assigned female adolescents who were receiving gender-affirming testosterone therapy and had total and free testosterone concentrations were included. The aims were to determine the fidelity of total and free testosterone concentrations with body mass index (BMI) and to characterize the association of testosterone concentrations with erythrocytosis. RESULTS: Of 68 subjects, 23% were overweight and 40% had obesity. Compared with the group with a healthy BMI, patients with obesity had significantly lower total testosterone concentrations. Free testosterone concentrations did not vary based on BMI category. BMI z-score was inversely correlated with total testosterone, sex hormone-binding globulin, and albumin concentrations but not with free testosterone concentrations. Four percent of subjects developed erythrocytosis (e.g., hematocrit >50%). Neither BMI nor testosterone concentrations were correlated with hematocrit. CONCLUSIONS: Free testosterone concentrations are an adjunctive tool in the monitoring of gender-affirming testosterone therapy. In addition, the development of erythrocytosis is independent of serum testosterone concentration and warrants further study to understand factors that may predispose patients to developing secondary polycythemia.