Here are some collections of scientific studies pertaining to transgender people:
Transsexual and Intersex Gender Identity
A curated list of brain-related studies, managed by a rocket scientist who’s an intersex trans woman.
What causes transsexualism?
Studies on the hormonal and genetic causes of transsexualism, including both human and animal studies
Treatments for gender dysphoria
Studies looking into the efficacy of hormone therapy and surgery in resolving gender dysphoria
Review of 52 studies, by Cornell University
Google doc: Transsexualism
A very detailed and organised Google Sheets database with links to thousands of studies on all aspects of trans issues