Autoimmune thyroiditis in paediatric thyroid cancer: clinical features and outcomes.

[OBJECTIVE] To evaluate clinical characteristics and survival outcomes of paediatric differentiated thyroid carcinoma (paedDTC) in patients with and without autoimmune thyroiditis (AIT).

[DESIGN] Retrospective cohort analysis of paedDTC patients with and without AIT, prospectively enrolled in the German Malignant Endocrine Tumour Registry between 1997 and 2024.

[METHODS] We analysed data from paediatric patients (<18 years) with histopathologically confirmed DTC, stratified by AIT status. Variables included demographics, tumour characteristics, lymph node involvement, and metastasis. Survival outcomes were assessed using Kaplan-Meier estimates.

[RESULTS] Among 401 evaluable patients, 16.0% had AIT. AIT patients were significantly older at diagnosis (P < .001) and showed a non-significant tendency toward higher female predominance (P = .098). Patients with AIT demonstrated significantly lower rates of distant metastasis (7.5% vs. 22.2%, P = .014) compared with those without AIT. No significant differences were observed in tumour size, capsule invasion, lymphatic or vascular invasion, soft tissue infiltration, or multifocality. A tendency toward improved event-free survival (EFS) was observed in AIT patients (87.5% vs. 72.1%, P = .255), while overall survival (OS) was comparable (91.8% vs. 86.0%, P = .167) during the study period. Among patients with known AIT history, paedDTC was significantly more frequently diagnosed through surveillance compared with non-AIT patients (44.2% vs. 28.3%, P = .022).

[CONCLUSIONS] AIT was present in 16% of paedDTC patients and was associated with reduced distant metastasis. A tendency toward improved EFS but similar OS during the follow-up period was observed. Earlier detection through surveillance in AIT patients may contribute to these findings, warranting further investigation into underlying biological mechanisms.