14 Chapter 1 (at least Tanner stage 2) (Coleman et al., 2022; Hembree et al., 20173). Any coexisting psychological, social, and/or medical problem that could interfere with the assessment or treatment should be addressed, with the underlying idea that the adolescent’s functioning and situation are stable enough to assess gender dysphoria/gender incongruence and to undergo treatment. Additionally, the adolescent should have sufficient cognitive capacity to give informed consent to treatment, after being informed about the effects and possible side effects of PS. Depending on the legislation in the respective country, the legal representative(s), which are in most cases the parent(s), should give informed consent too, or on behalf of the adolescent. In addition, the parent(s) or other caretaker(s) is/are involved in the care provided to the transgender adolescent, and are offered support if desired (de Vries & Cohen-Kettenis, 2012). Table 3 shows the diagnostic criteria for treatment with PS for adolescents. Table 3. Diagnostic criteria for treatment with puberty suppression for adolescents * Adolescents are eligible for treatment with puberty suppression if: • The adolescent has demonstrated an intense and long-lasting pattern of gender dysphoria or gender nonconformity (whether expressed or suppressed) • The gender dysphoria emerged or worsened with the onset of puberty • Any concurrent psychological, social, and/or medical issues that could interfere with the treatment (for example, that may compromise compliance with the treatment) have been addressed, such that the functioning and situation of the adolescent are stable enough to start the treatment • The adolescent is having sufficient mental capacity to give informed consent to the treatment • The adolescent and/or parent(s)/other caretaker(s) (depending on the adolescent’s age and local laws) has/ have given informed consent after being informed about the effects of the treatment and fertility preservation options • The parent(s)/other caretaker(s) is/are involved and supporting the adolescent throughout the treatment process • A paediatric endocrinologist or other clinician with experience in the assessment of puberty agrees with the indication of the clinician to start puberty suppressing treatment • The adolescent’s puberty has started (Tanner stage ≥ G2/B2) • The adolescent had no medical contraindications to treatment with puberty suppression * Coleman et al., 2022; Hembree et al., 2017 In addition, the two established international transgender guidelines recommend the use of gender-affirming hormones (GAH; testosterone or oestrogen) after the start with treatment with PS for adolescents who still show gender dysphoria at about the age of 15-16. In the case of compelling reasons, treatment with GAH could be initiated for adolescents prior to the age of 16 years (Hembree et al., 2017). Surgery, for example 3 Very recently, Coleman et al., published a revised version of one of these guidelines, which resembles to a large extend the earlier version but also includes some significant changes regarding recommended care for transgender and gender non-conforming children and adolescents of which it is as yet not clear how they will affect clinical practice (2022). The research described in chapters 2 to 9 of this thesis is conducted at the time of the 7th version of the Standards of Care. Therefore, these chapters refer to the 7th version of these guidelines, while chapters 1 and 10 refer to the 8th version. Nevertheless, it is worth noting that the changes to the 8th version of the Standards of Care would not have changed the conclusions of the research described in chapters 2 to 9.