125 Perceptions on the function of puberty suppression of transgender adolescents who continued or discontinued treatment, their parents, and clinicians 7 INTRODUCTION International guidelines recommend treatment with puberty suppression (PS; using gonadotropin-releasing hormone analogues (GnRHa)) for transgender adolescents if certain criteria are fulfilled (Coleman et al., 2012; Hembree et al., 2017). It is recommended to start treatment after adolescents first exhibit physical changes of puberty (at least Tanner stage 2), in order to suppress further development of secondary sex characteristics in a reversible manner. According to these guidelines, one of themain reasons to start PS is to ‘pause’ puberty to expand the diagnostic phase so that adolescents have ‘extra’ time to explore their options and think about pursuing subsequent gender-affirming medical treatment (GAMT) with irreversible effects. A second reason to start PS in early puberty is that the physical outcome may be more satisfactory compared to PS in later stages of puberty and some surgeries such as mastectomy may not be necessary or less invasive because development of secondary sex characteristics is prevented (van de Grift et al., 2020). This may be a life-long advantage for adolescents (Cohen-Kettenis & van Goozen, 1997). In addition, PS can be used in adolescents in later stages of puberty to prevent facial hair growth in transgirls (assigned male at birth, with a female gender identity) and to stop menses in transboys (assigned female at birth, with a male gender identity) (Hembree et al., 2017). Treatment with PS has been shown to improve psychological functioning of adolescents in various domains (de Vries et al., 2011a; van der Miesen et al., 2020). Experiencing full endogenous puberty might impair well-being and healthy psychological functioning (Hembree et al., 2017). However, the positive effects of PS need to be weighed against possible drawbacks. Some of the long-term effects of PS are still unknown (Biggs, 2021; Giordano & Holm, 2020; Harris, Tishelman, Quinn, & Nahata, 2019). A potential negative impact on cognitive, physical and psychosocial development has been mentioned, for example the risk of impaired fertility (Chen et al., 2020; Harris, Kolaitis & Frader, 2020; Laidlaw et al, 2019b). Furthermore, concerns have been raised that preventing exposure to sex hormones and disrupting pubertal and sexual development may alter the course of gender identity development and may prevent spontaneous resolution of gender dysphoria or the recognition of oneself as homosexual rather than transgender (Korte et al., 2008; Vrouenraets et al., 2015). Furthermore, in a qualitative interview study, clinicians stated that they have concerns about the lack of long-term data on some possible side effects of treatment with PS (Vrouenraets et al., 2015). Additionally, certain options for genital surgery may not be available to those who started PS early in puberty necessitating the use of more invasive alternatives (van de Grift et al., 2020). Besides, there are worries about the risk of regret, since gender identity might fluctuate during adolescence (Vrouenraets et al., 2015). The results of a qualitative interview study with transgender adolescents showed that some adolescents themselves also had some hesitations regarding early medical treatment. They reported doubts, for example, about the ability of adolescents to make informed decisions with regard to medical treatment at the age of 12 or younger (Vrouenraets et al., 2016).