There is no scientific consensus as to what causes gender dysphoria.
But doesn't mean it hasn't been studied.
The hormone issue pointed towards homosexuality, as much as it did anything else. An effeminate brain, if you like. It certainly doesn't support a female brain in a male body.
This is an article which sets out the four types of gender dysphoria as they are known.
Including rapid onset gender dysphoria (ROGD) and autogynephilia.
The first of which trans pressure groups flatly deny, despite gender clinics talking extensively about it (See Polly Carmichael of the Tavistock speaking at a recent conference).
And autogynephilia is evidently prevalent. But as a fetish it undermines the whole 'born in the wrong body' narrative. So it's also flatly denied. Despite there being numerous websites devoted to it.
See #transgirls and #girlslikeus.
It's a little difficult to deny when Stonewall's own website includes transvestism as part of the official trans umbrella.
Trans-pressure groups like GIRES, Gendered Intelligence and Mermaids are very one sided.
Polly Michael pointed towards Transgender Trend, in terms of a resource (for parents). Which was very telling.