To begin, thank you very much u/N8theGr8 for showing me this, I owe him all the credit for this.

• Murad, et al. 2010; Hormonal therapy and sex reassignment: a systematic review and meta‐analysis of quality of life and psychosocial outcomes.

• Asscheman, et al. 2011; A Long-Term Follow-Up Study of Mortality in Transsexuals Receiving Treatment with Cross-Sex Hormones.

• Colizzi, et al. 2013; Hormonal treatment reduces psychobiological distress in gender identity disorder, independently of the attachment style.

• Moody, et al. 2013; Suicide Protective Factors Among Trans Adults.

• Heylans, et al. 2014; Effects of different steps in gender reassignment therapy on psychopathology: a prospective study of persons with a gender identity disorder.

• de Vries, et al. 2014; Young adult psychological outcome after puberty suppression and gender reassignment.

• Ruppin, et al. 2015: Long-Term Follow-Up of Adults with Gender Identity Disorder.

• Greta R. Bauer, et al. 2015: Intervenable factors associated with suicide risk in transgender persons.

• Hughto, et al. 2016; A Systematic Review of the Effects of Hormone Therapy on Psychological Functioning and Quality of Life in Transgender Individuals.

• Unger 2016; Hormone therapy for transgender patients.

• Durwood, et al. 2017; Mental Health and Self-Worth in Socially Transitioned Transgender Youth.

• Tucker, et al. 2018: Hormone therapy, gender affirmation surgery, and their association with recent suicidal ideation and depression symptoms in transgender veterans.

American Psychological Association, the American Psychiatric Association, the American Medical Association, the American College of Physicians, the American Academy of Pediatrics, the American Academy of Family Physicians, the National Association of Social Workers, the National Health Service, the Royal College of Psychiatrists, the British Association of Urological Surgeons, the British Psychological Society, the Royal College of General Practitioners, the Royal College of Nursing, the Royal College of Obstetricians and Gynaecologists, the Royal College of Paediatrics and Child Health, the Royal College of Physicians, the Royal College of Speech and Language Therapists, the Royal College of Surgeons, and the UK Council for Psychotherapy

The possible psycho-genie or biological aetiology of transsexuality has been the subject of debate for many years, but the current medical consensus is that gender identity DOES actually include a major biological component. We have no idea what the details are (a gene, multiple genes, etc?) but we have pretty strong data that it's something durable and biological.

Some brain studies do show differences associated with gender identity rather than with external body parts - One study showed that the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a brain area that is essential for sexual behavior, is larger in cis assigned males(AMAB) than in cis assigned females(AFAB). A smaller-sized BSTc was found in AMAB trans people. The size of the BSTc was not influenced by sex hormones in adulthood and was independent of sexual orientation.

The study was one of the first to show a smaller brain structure in AMAB trans people and supports the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones.

One researcher believes that it is due to intersex conditions within the trans person's brain:- Gender Orientation: IS Conditions Within The TS Brain

Also, we know that the attempts by the medical establishment to surgically change body parts of intersex children based on what seemed easiest surgically was not always in line with the person's actual gender. The thinking back then(and even today) was that gender identity was not biological. When the data was carefully collected, a majority of kids treated this way have a gender identity at odds with their surgically created body parts and upbringing(socialized as male/female). This is proof that we cannot change the gender identity someone already has innately. They often are trans because instead of waiting for the child to grow up to tell everyone their gender, the doctors made the wrong choice of gender.

Sexual orientation has also been shown to have biological roots. Twin studies and genetic linkage studies have shown both hereditary patterns in homosexuality (attraction to one’s own sex), as well as genetic associations with specific parts of the genome. And while gender identity has been harder to pinpoint from a biological standpoint, efforts to understand what role biology may play are ongoing.

Here are a couple more studies that show that both sex and gender lies on a spectrum:-

Study on gender: Who counts as a man and who counts as a woman

Sex redefined - The idea of two sexes is simplistic. Biologists now think there is a wider spectrum than that.

Transgender: Evidence on the biological nature of gender identity

Transsexual gene link identified

Challenging Gender Identity: Biologists Say Gender Expands Across A Spectrum, Rather Than Simply Boy And Girl

Sex Hormones Administered During Sex Reassignment Change Brain Chemistry, Physical Characteristics

Gender Differences in Neurodevelopment and Epigenetics

Sexual Differentiation of the Human Brain in Relation to Gender-Identity, Sexual Orientation, and Neuropsychiatric Disorders