https://pubmed.ncbi.nlm.nih.gov/24344788/ https://link.springer.com/article/10.1007/s10508-009-9551-1 https://mayoclinic.pure.elsevier.com/en/publications/hormona... https://link.springer.com/article/10.1007/s10508-014-0453-5 https://www.researchgate.net/publication/23553588_Long-term_... http://europepmc.org/article/med/25690443 https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2265.... https://pubmed.ncbi.nlm.nih.gov/29463477/

If by "quite new and unsupported" you mean "pioneered in the 1920s, well-established for many decades, backed by dozends of studies, and successfully applied to hundreds of thousands of patients worldwide", then yes - it is indeed quite new and unsupported.

If you want to read just one study, why not start with the last one? The Amsterdam Cohort consists of 6,793 people since 1972. That should be plenty to start with.

If you want even more, I recommend reading the "Standards of Care for the Health of Transgender and Gender Diverse People". Currently in its eighth edition, it describes the entire process, with hundreds of additional studies cited.