The study looked at 9,916 trans women on estrogen and found that testosterone levels drop when estradiol goes up, but the two hormones don’t move together in a simple way. Things like having the testicles removed, being older, or taking spironolactone change testosterone levels but not estradiol. Testosterone also tracks with LH, while estradiol does not. So, for people doing gender‑affirming hormone therapy, checking testosterone (and LH if you don’t have testicles) is more useful than just checking estradiol.

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. 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. 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. 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.