A true medical or biological condition would be intersex. Intersex people have true biological gender abnormality. incomplete male or female genitialia. sometimes having both. May outwardly have all the typical features of one gender, but the the genitalia of the other. There are certain medical conditions which may cause a person to have these issues. Klinefelter syndrome often attributes to an intersex condition. People with Kinefelter syndrome often never hit puberty. Have un-descended testicles. extremely small genitialia. A much higher pitched voice than a typical male will have. Often will have some gynocomastia. (male breasts.) These would be medical conditions, which also may cause some people to experience being transgender, but not always.
Dysphoria, which you say you suffer from, is often because the brain tells us one thing, while our bodies are the opposite. Transgender people often will have nothing about their body in a biological sense that has anything wrong with it, apart from any normal medical condition. Their hormone levels will be in the typical range, males will hit puberty at a typical age. All of the usual changes resulting in puberty will take place. It is those changes which often make the dysphoria very difficult, along with the expectations of society, the identity others see us as. Our genitalia which we may feel is alien to us. Many who are TS have said to have felt that their genitalia is simply wrong, and they hate it, along with all of the rest of their characteristics.
There is very little which can be found even in autopsies which will indicate anything about a person that would show being transgender. There is no blood test, or any other physical test which will show a transgender condition. I would agree and will assume most who are transgender will agree that something does occur in pre birth. Transgender does not discriminate based on race or geography. There is nothing in the water which causes it. Nothing which has ever been found to make or more or less likely during pregnancy. There are no predictors to being transgender.
What that something really is??? Some sort of wiring that causes transgender people to identify or relate to the opposite biological sex. It has in the past been considered a mental illness. Transvetic fetishism, or transexualism. Now thankfully the DSM has removed transgender and sub terms from it being considered a mental illness. Transgender people sometimes suffer from depression or other mental illness as a result of the difficulties of being transgender.
There are crossdressers who occasionally dress, never in anyway feel they are women, or men for FTM. They always internally identify as male, even when dressed. some do not consider themselves to be transgender, and who is anyone to argue that? It really is a personal thing, and it isn't a diagnosis, it is just an acceptance of who you are.