Context Testosterone (T) or estradiol (E2) are administered to suppress gonadal function in female-to-male (FTM) and male-to-female (MTF) transgender patients. How often sex steroid cause adequate suppression without GnRH agonist (GnRHa) or progestin therapy has not been reported. Objectives 1) To determine how often T and E2 therapy alone can effectively suppress gonadal function in MTF and FTM transgender patients. 2) To determine the frequency and range of serum E2 levels above the normal male range in FTM patients receiving T therapy. Design Retrospective cohort study. Setting Outpatient reproductive endocrinology clinic at an academic medical center. Patients 65 FTM and 33 MTF patients were included who were > 18 years old and not receiving a progestin or GnRHa. Intervention FTM patients were receiving T through injections or gel. MTF patients were receiving oral or subcutaneous E2. Main Outcome Measurements In FTM patients the indicator of ovary suppression was amenorrhea. In MTF patients the indicator of testes suppression was T levels <50 ng/dL. Results Median serum total T level for FTM patients was 712 ng/dL (range, 370-1164 ng/dL). On T therapy alone, 90.8% of patients achieved amenorrhea. 49.2% of patients had serum E2 levels above the normal range for women. For MTF patients, the median serum E2 level was 129.2 pg/mL (range, 75-197 pg/mL). On E2 therapy alone, 84.8% of MTF patients had adequate suppression of testicular function. Conclusions T and E2 therapy are usually effective without progestin or GnRHa therapy to suppress gonadal function in transgender patients. Progestin and/or GnRHa therapy should only be initiated in those patients who do not have adequate gonadal suppression on optimized doses of T or E2 alone.

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