This is what the Tavi GIDS publishes on its evidence base page which you can find here:
gids.nhs.uk/evidence-base
There are different options available around physical interventions. Some young people may decide not to access any form of physical intervention. For those who do, few people will choose to access the full range of physical treatments. Figures from the Scottish Trans Alliance Survey (2008) show that 24% (17/71) of the transgender respondents have not transitioned on a permanent basis from female-to-male or from male-to-female, but are either still living partly or fully as the gender they were labelled at birth or alternatively are living in a non-binary gender expression which is not clearly male or female.
Hormone treatment
Although hormone blockers and cross-sex hormone treatment are recommended in young people with GD and widely used across the board, it should be noted that the research evidence for the effectiveness of any particular treatment offered is still limited.
A Dutch research programme indicates that a treatment protocol including puberty suppression followed by cross-sex hormones and gender reassignment surgery, leads to improved psychological functioning in a selected group of transgender adolescents, who had persistent GD from childhood, lived in a supportive environment and had no serious co-morbidities. If the young people did not show persistent GD from childhood, live in a supportive environment or if they had serious co-morbidities, assessment was prolonged (de Vries et al 2014). The hormone blocker alone does not seem to alleviate feelings of GD, however, it does have a positive impact on adolescents’ psychological well-being by putting their pubertal development on hold (de Vries et al 2010). The Dutch authors conclude: ‘Clinicians should realize that it is not only early medical intervention that determines this success, but also a comprehensive multidisciplinary approach that attends to the adolescents’ GD as well as their further well-being and a supportive environment’ (de Vries et al 2014). Additionally, having good peer relationships and engaging in social interaction with other transgender people have both been shown to help build resilience and improve psychological well-being (Testa, Jimenez & Rankin, 2014; De Vries et. al., 2015).
Safety concerns
Safety concerns remain regarding the impact of physical interventions. Although puberty suppression, cross-sex hormones and gender reassignment are generally considered safe treatments in the short term, the long-term effects regarding bone health and cardiovascular risks are still unknown (Cohen-Kettenis & Klink, 2015; Klink et al., 2015,).
As with a lot of the literature around trans kids - there is a subset of teenagers who have been convinced they were "born in the wrong body" from a very young age. This group is allowed to go through Tanner stage 1, and if the GD persists can be considered for puberty blockers. The evidence cited above is for this group "persistent GD from childhood".
I have no doubt that the Tavi doesn't treat kids lightly, and I expect come under huge pressure to do it more. I am certain they don't treat an 11 year old who has only a few months before revealed his GD to his parents.