There is limited evidence that medical transition leads to positive outcomes.
A number of different studies have noted the paucity of good quality evidence for transition.
An Australian paper  states that most available evidence indicating positive outcomes for gender reassignment is of poor quality.
A German study  “found insufficient evidence to determine the efficacy or safety of hormonal treatment approaches for transgender women in transition”, adding that “[t]his lack of studies shows a gap between current clinical practice and clinical research.”
A British review  conducted by the National Institute for Health and Care Excellence (NICE) graded certainty of evidence for puberty blocker use as “very low” in every category, including impact on gender dysphoria, mental health, body image, global functioning, psychosocial functioning, cognitive functioning, bone density and adverse effects.
A chapter  in an edited volume details the low evidence base for treatment pathways employed at the UK’s Gender Identity Development Service, demonstrating how negative evidence was “ignored or suppressed”.
Finally, a systematic review  commissioned by the World Professional Association for Transgender Health (WPATH) to “systematically review the effect of gender-affirming hormone therapy on psychological outcomes among transgender people” noted that, in some areas, there was low quality or insufficient evidence.
 D’Angelo, R. (2018). Psychiatry’s ethical involvement in gender-affirming care. Australasian Psychiatry 26 (5): 460-463. [Link]
 Haupt, C., Henke, M., Kutschmar, A., Hauser, B., Baldinger, S., Saenz, S.R. & Schreiber, G. (2020). Antiandrogen or estradiol treatment or both during hormone therapy in transitioning transgender women. Cochrane Database of Systematic Reviews 11. [Link]
 National Institute for Health and Care Excellence (2021). Evidence review: Gonadotrophin releasing hormone analogues for children and adolescents with gender dysphoria. National Institute for Health and Care Excellence (NICE); NHS England; NHS Improvement. [Link]
 Biggs, M. (2019). The Tavistock’s Experiment with Puberty Blockers. In: Moore, M. & Brunskell-Evans, H. (eds.). Inventing Transgender Children and Young People. Cambridge Scholars Publishing. [Link]
 Baker, K.E., Wilson, L.M., Sharma, R., Dukhanin, V., McArthur, K. & Robinson, K.A. (2021) Hormone Therapy, Mental Health, and Quality of Life Among Transgender People: A Systematic Review. Journal of the Endocrine Society 5 (4). [Link]