The EuRREB Core Endocrine Registry and its Potential for Studying Transition of Care in Endo-ERN Centres.

Natural history registries can address knowledge gaps and support the development of best practice guidelines to improve outcomes in rare conditions. The European Registries for Rare Endocrine and Bone conditions (EuRREB) Core Registry is a standardised, multi-national platform designed to collect harmonised clinical data across rare endocrine disorders and facilitate longitudinal real-world analyses. The aim of this study was to describe data collected in the Core Registry across eight endocrine main thematic groups (MTGs) during the first six years following its launch in 2019, with particular focus on the transition period, defined as 14-25 years of age. Forty-eight centres from 20 European and non-European countries contributed data, of which 33 (69%) were members of the European Reference Network on rare endocrine conditions (Endo-ERN). A total of 3225 unique patients were registered, with a median age of 37 years. Overall, the Hypothalamic and Pituitary MTG accounted for the largest number of records (n=1548). The highest proportion of patients in the transition age group was observed in the Sex Development and Maturation MTG (66%, 445/677), followed by Thyroid (33%, 86/260) and Growth and Genetic Obesity (32%, 42/130). A total of 303 patients had an active platform account, and 85 completed patient-reported outcome measures. Clinicians completed a total of 2390 condition-specific outcomes These findings demonstrate the capacity of the Core Registry to generate real-world insights across rare endocrine conditions and age groups. The observed transition-age representation highlights an opportunity to strengthen structured pathways and support continuity of care across the paediatric-adult interface.