Choice of Hormone Assay to Monitor Feminizing Gender-Affirming Hormone Therapy

Objective: Studies have shown variability in the correlation among testosterone (T) concentrations, estradiol (E2) concentrations, and other clinical parameters to monitor response to feminizing gender-affirming hormone therapy (GAHT). We aimed to determine the degree to which data support the use of serum T, serum E2, luteinizing hormone (LH), or follicle stimulating hormone to monitor feminizing GAHT with the goal of decreasing the effect of endogenous androgens. Methods: We conducted a cross-sectional analysis of T and E2 concentrations in 9916 transfeminine individuals prescribed estradiol to examine the association between individuals’ most recent serum T, E2, LH, and follicle stimulating hormone concentrations in the context of feminizing GAHT treatment. Results: Changes in T concentrations were inversely correlated with changes in E2 concentrations (P < .001). However, orchiectomy, age, and the use of spironolactone were associated with changes in T, but not E2 concentrations (P < .001). Changes in T concentrations were also correlated with changes in LH (P < .05). Such correlations were not demonstrated with E2 concentrations. Conclusions: To monitor feminizing GAHT, it may be reasonable to favor target T concentrations over E2 concentrations in patients with testes and to consider LH concentrations in patients without testes who are not taking gonadotropin-releasing hormone agonists.