A Study of Selpercatinib (LY3527723) in Participants With RET-Mutant Medullary Thyroid Cancer

* At least 18 years of age (participants as young as 12 years of age will be allowed if permitted by local regulatory authorities). * Histologically or cytologically confirmed, unresectable, locally advanced and/or metastatic MTC and no prior history of treatment with kinase inhibitors for advanced/metastatic disease. * Radiographic progressive disease per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 at screening compared with a previous image taken within the prior 14 months as assessed by the BICR. Participants with measurable or non-measurable but evaluable disease are eligible; however, participants with non-measurable disease may not have disease limited to bone sites only. * A defined/acceptable RET gene alteration identified in a tumor, germline deoxyribonucleic acid (DNA) or blood sample. * Tumor tissue in sufficient quantity to allow for retrospective central analysis of RET mutation status * Eastern Cooperative Oncology Group performance status score of 0 to 2. * Adequate hematologic, hepatic, and renal function and electrolytes. * Men and women of childbearing potential must agree to use a highly effective contraceptive method during treatment with study drug and for 4 months following the last dose of study drug. * Ability to swallow capsules. Exclusion Criteria: * An additional validated oncogenic driver in MTC if known that could cause resistance to selpercatinib treatment. Examples include, but are not limited to RAS or BRAF gene mutations and…