Finnish nationwide cohort of all under-23-year-old gender-referred individuals between 1996 and 2019 (n = 2 083) and 16 643 matched controls. Cross-tabulations with X2 statistics and Cox regression were used to analyse the data.

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Gender-referred adolescents showed significantly higher psychiatric morbidity than controls both before (45.7% vs. 15.0%) and ≥ 2 years after referral (61.7% vs. 14.6%). Those referred after 2010 had greater psychiatric needs than earlier cohorts, both before (47.9% vs. 15.3%) and ≥ 2 years after (61.3% vs. 14.2%) referral. Among adolescents who underwent medical gender reassignment, psychiatric morbidity increased markedly during follow-up—rising from 9.8% to 60.7% in feminising gender reassignment and from 21.6% to 54.5% in masculinising gender reassignment. After adjusting for prior psychiatric treatment, all gender-referred adolescents had similarly elevated risks of psychiatric morbidity, with hazard ratios approximately three times higher than female controls and five times higher than male controls.