Restor. I-131 Upt. + Selpercatinib in RET F-P RAI-R TC

Inclusion Criteria: * Participants must have histologically- or cytologically-confirmed follicular-derived nonanaplastic thyroid cancer that is metastatic and/or unresectable AND harbors a known oncogenic RET gene fusion, performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified or equivalently accredited diagnostic laboratory. * Tumor tissue or liquid biopsy-based next-generation sequencing (NGS), quantitative polymerase chain reaction (qPCR), and fluorescence in situ hybridization (FISH) for RET gene fusion detection will be permitted. * Participants ≥ 18 years of age must have measurable disease, per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. \-- Participants 12 to \<18 years of age may enroll with either evaluable (i.e. anatomically visible tumor on cross sectional imaging, but tumors do not need to be \>1 cm) or measurable disease per RECIST v1.1. * Participants should have no single tumor deposit exceeding 4.0 cm in the greatest dimension. * Participants must have RAI-refractory disease, as defined by: * The malignant tissue does not concentrate RAI on a whole body scan following radioiodine administration * The tumor tissue has lost the ability to concentrate RAI on a whole body scan following radioiodine administration after previous evidence of uptake at any earlier timepoint, * RAI is taken up in some but not all tumor deposits on a whole body scan following radioiodine administration and/or * There is progressive disea…