The Gender Identity Development Service (GIDS), based at the Tavistock and Portman Trust in North London, was set up initially to provide - for the most part - talking therapies to young people who were questioning their gender identity. But in the last decade GIDS has referred more than a thousand children, some as young as nine years old, for medication to block their puberty. In the same period, the number of referrals has exploded, increasing thirty-fold, while the profile of the patients has changed, from largely pre-pubescent boys to mostly adolescent girls, who are often contending with other difficulties.