Testosterone use in female mice does not impair fertilizability of eggs: Implications for the fertility care of transgender males

STUDY QUESTION: Does testosterone use in females affect reproductive potential, particularly with regard to the production of fertilizable gametes? SUMMARY ANSWER: Testosterone cypionate injections given to post-pubertal female mice caused virilization and ovaries were smaller than control ovaries, but ovaries were still responsive to hormonal stimulation and produced fertilizable eggs when superovulated. WHAT IS KNOWN ALREADY: Studies to examine the effects of testosterone on reproductive potential in transgender males are lacking. Recently, a model was developed that simulates many aspects of testosterone use in transgender males in order to look at reproductive effects of testosterone in female mice. This study found masculinizing effects on the mice but did not find significant deficits on the number of ovarian follicles; however, effects of testosterone use on ovarian stimulation and fertilizability of oocytes were not investigated. STUDY DESIGN, SIZE, DURATION: A total of 66, 6-week-old Hsd:NSA(CF-1) female mice and 6 Hsd:ICR (CD-1) mice were used for this study. Mice were injected subcutaneously with 400 μg testosterone cypionate or sesame oil once a week for 6 weeks and were either sacrificed a week after the 6th injection (active exposure group), or were sacrificed 6-7 weeks after the final testosterone injection (washout group). PARTICIPANTS/MATERIALS, SETTING, METHODS: Both active exposure and washout groups were further subdivided into 3 groups: unstimulated, eCG-stimulated, or eCG/hCG-stimulated. eCG-stimulated mice were sacrificed 44-48 hrs after eCG injection. eCG/hCG-stimulated mice were injected with eCG, followed 48 hrs later with hCG. Mice were sacrificed ~13-18 hrs after the hCG injection. Data collected included daily vaginal cytology, terminal hormone levels and ovary weights, ovarian histology, number of oocytes/eggs collected in each group, and cleavage to the 2-cell stage following in vitro fertilization. MAIN RESULTS AND THE ROLE OF CHANCE: Testosterone cypionate-treated mice had testosterone levels elevated to the level of male mice and ceased cycling. Ovaries were significantly smaller in testosterone-treated mice, but they contained normal cohorts of follicles and responded to gonadotropin stimulation by ovulating similar numbers of eggs that fertilized and cleaved in vitro. LIMITATIONS, REASONS FOR CAUTION: Our model treated female mice for only 6 weeks, whereas many transgender men use testosterone for many years before considering biological children. Importantly, a mouse system may not perfectly simulate human reproductive physiology. WIDER IMPLICATIONS OF THE FINDINGS: The current standard of care for transgender men who desire biological children is to cease testosterone therapy prior to ovarian stimulation, but the necessity for stopping testosterone is not known. Our model demonstrates that it is possible for testosterone-suppressed ovaries to respond to gonadotropic stimulation by producing and ovulating fertilizable eggs, thereby obviating the need for testosterone cessation prior to ovarian stimulation. In time, these results may provide insights for future clinical trials of fertility treatment options for transgender men.