Transition from paediatric to adult endocrinology is a challenge for adolescents, families and doctors. Up to 25% of young adults with chronic endocrine disorders are lost to follow-up ('drop-out') once the young adult moves out of paediatric care. Non-attendance and sub-optimal medical self-management can lead to serious and expensive medical complications. In a pilot study, adolescents suggested the use of e-technology to become more involved in the transition process. The investigators have designed and developed the YESS! game, a tool to help improve medical self-management in adolescents with chronic endocrine disorders. The hypothesis is that adolescents playing the YESS! game will show a larger increase in self-management score during the first year of transition and will have a lower drop-out rate at the adult endocrine outpatient clinic (OPC), compared to adolescents who do not play the game.

Rationale: Transition from paediatric to adult endocrinology is a challenge for adolescents, families and doctors. Up to 25% of young adults with chronic endocrine disorders are lost to follow-up ('drop-out') once the young adult moves out of paediatric care. Non-attendance and sub-optimal medical self-management can lead to serious and expensive medical complications. In a pilot study, adolescents suggested the use of e-technology to become more involved in the transition process. The investigators have designed and developed the YESS! game, a tool to help improve medical self-management in adolescents with chronic endocrine disorders. The hypothesis is that adolescents playing the YESS! game will show a larger increase in self-management score during the first year of transition and will have a lower drop-out rate at the adult endocrine outpatient clinic (OPC), compared to adolescents who do not play the game.

Objective: 1.To improve medical self-management. 2.To prevent drop-out from the adult outpatient clinic. Study design: multicentre randomized controlled trial Study population: 160 transition patients from 15 to 20 years old from the participating countries Spain, The United Kingdom, Belgium and the Netherlands. Patients are diagnosed with a chronic endocrine disorder. Intervention: The study consists of 4 study arms: receiving the YESS! game and toolkit (group YT), receiving the control game and toolkit (group GT), receiving the toolkit only (group T) and receiving regular transition care (group O). Every group will receive regular transition care. The transition toolkit consists of paper cards with assignments, ideas and tips regarding medical self-management. Main study parameters: Primary outcome: the Self-management and Transition to Adulthood with Rx (=treatment) (STARx) questionnaire score 12 months after inclusion in group YT compared to group O. Secondary outcome: the STARx questionnaire score 6 months after inclusion in group YT compared to the other study groups, the STARx questionnaire score 12 months after inclusion in group YT compared to group GT and T and the drop-out rate to the adult outpatient clinic in the first year after the last visit to the paediatric endocrinologist (i.e. one year after the moment of transfer) in group YT compared to groups GT, T and O.

Nature and extent of the burden and risks associated with participation: The participants are not exposed to any risks. The YESS! and the control game are safe apps played on a mobile phone or tablet. The burden consists of filling out the STARx questionnaires and playing the YESS! or control game. The questionnaires will be filled out online at home at the start of the study and after 6 and 12 months. Every 6 months the subject has an appointment at the outpatient clinic. The participant can play the YESS! game for a maximum of 15 minutes a day to prevent game addiction. The control game could be played for an unlimited amount of time, but will unlikely cause game addiction since the game is not considered challenging and exciting enough.

Inclusion Criteria:

• Aged 15 to 20 years old.
• Diagnosed with congenital adrenal hyperplasia, hypogonadotropic hypogonadism, Turner syndrome, Klinefelter syndrome, growth hormone deficiency, combined pituitary hormone deficiency, Androgen insensitivity syndrome, thyroid dysgenesis or Addison's disease

Exclusion Criteria:

• Lack of a mobile phone or tablet.
• Intellectual disability or language barrier leading to inability to use the YESS! game or the control game.

The IPD has not been finalized yet. The investigators intent to share the full anonymized dataset, study protocol and statistical analysis plan upon request after publication of the results. Informed consent forms will not be shared."