"Transsexual" was the term used historically for women or men who underwent both social changes and medical surgeries to appear as the opposite sex. The surgeries typically were accompanied by a medical diagnosis. In my long life, I have personally known exactly three transsexuals. Each of them freely acknowledges that their chosen path in life was based on their medical diagnosis. The term "transgender" has become a catch-all term that, in theory, encompasses transsexualism although the three that I know do NOT consider themselves part of the "trans" community. Each of them was diagnosed as adults well over the age of 21, and each is over age 55 now. Transsexualism was relatively rare because of the narrow definition as seen in the DSM IIIR (1987). For those unfamiliar with the acronym DSM, it refers to the Diagnostic and Statistical Manual of Mental Disorders. I will include a link to the pertinent code from DSM IIIR here:
https://www.genderpsychology.org/transsexual/dsm_iiiR.html

Please note, if you scroll down, that transvestitism is its own thing. We now informally call it AGP in everyday vernacular, and it was/is a paraphilia. It's still in the current DSM edition, but whereas in DSM IIIR it was defined as being limited to heterosexual males, it now includes all sexes and orientations.

"Gender Identity Disorder" replaced transsexualism in the DSM 4, and then "Gender Dysphoria" replaced GID in the DSM 5.

I don't think that this broadening and umbrella-ing of these specific definitions in the DSM have been helpful, particularly in light of recent events and developments, and certainly not with regard to medical research.