Abstract
BACKGROUND: Brain metastases are an exceptionally rare and diagnostically challenging manifestation of differentiated thyroid carcinoma (DTC) in pediatric and young adult patients. The authors present the case of a 21-year-old woman with a history of follicular variant papillary thyroid carcinoma who developed multiple intracranial lesions concerning for metastases. OBSERVATIONS: The patient had previously undergone total thyroidectomy and radioactive iodine ablation, with normal serum thyroglobulin and no known systemic spread. Brain biopsy was deferred given the location of the lesions and the risk associated with biopsy. LESSONS: Despite the rarity of brain metastases in pediatric and young adult populations with DTC, especially in the absence of concurrent extracranial disease, this case highlights the importance of thorough neurological evaluation and consideration of CSF thyroglobulin as a potential marker. This case emphasizes the need for vigilance in surveillance and an alternative to brain biopsy when intracranial metastasis is suspected. https://thejns.org/doi/10.3171/CASE25386.