SUN-135 Effects of Testosterone Treatment on the Morphology of Female Internal Genitalia Assessed by Transabdominal Ultrasound in Trans Boys

Abstract Disclosure: P.B. Norup: None. K. Sundberg: None. L.N. Jensen: None. M. Haahr: None. A. Giraldi: None. A. Pagsberg: None. A. Juul: None. M. Hilden: None. K.M. Main: None. Background: International guidelines for the healthcare of transgender youth include the possibility of hormone therapy. Recent concerns have been raised about testosterone treatment in trans men and its effects on their fertility potential as well as the risk of endometrial hyperplasia and cancer. To our knowledge, current studies are conducted in adult transmasculine individuals. Aim: To investigate the effects of testosterone on the internal female genitalia assessed by transabdominal ultrasound (TAUS) in an unselected population of trans boys who initiated hormone therapy before the age of 18 years. Methods: Trans boys were referred to a TAUS scan after approximately one year of testosterone treatment (n=48). 2D ovarian and uterine TAUS were conducted by experienced gynecologists using the Voluson E10 Ultrasound System (GE Healthcare Medical Systems, Zipf, Austria) and a multifrequency transabdominal probe (C2-9). All TAUS findings are given as median (10th, 90th percentiles). Hormone serum concentrations are presented as median (IQR). Results: At the start of testosterone treatment the median age was 16.9 years. Some of the trans boys had prior GnRH analog monotherapy (n= 31), and Tanner stage ranged from 3 to 5 before any treatment. Self-reported age at menarche was 12 years (range 9 - 15). The median age at TAUS was 18.0 after a median duration of testosterone treatment corresponding to 15.0 months. Serum concentrations were 0.8 IU/L (0.2 - 3.1) for LH, 3.2 IU/L (1.2 - 6.2) for FSH, 18.3 nmol/L (12.2 - 26.9) for testosterone, and 104.8 pmol/L (76.4 - 134.8) for estradiol. The mean ovarian volume, in trans boys with both ovaries visualized, was 8.9 cm3 (3.4, 13.8). The median number of small follicles was 3 (1, 5), and the largest follicle measured 6.0 mm (3.3, 11.5). Uterine volume was 77.0 cm3 (37.9, 104.9), uterine length was 6.7 cm (5.0, 7.8), and endometrial thickness was 4.0 mm (2.2, 6.0). Conclusion: Our results indicate that testosterone monotherapy may not fully suppress the pituitary-ovarian axis in trans boys, and the aromatization of testosterone to estradiol may be sufficient to stimulate the endometrium. This could result in cyclic pain and breakthrough bleeding. Additionally, this may have implications for contraception in sexually active adolescents. Presentation: Sunday, July 13, 2025