The purpose of this research study is to determine if certain markers in your blood are increased over the duration of your chemotherapy, especially as you get sicker, such as developing a severe infection (sepsis) or severe respiratory failure (acute respiratory distress syndrome).

This study involves taking a study drug called selpercatinib. The overall goal of this study is to find out how effective the study drug is in combination with radioactive iodine (I-131) in treating people with thyroid cancer. The study drug will be given in combination with treatments you will receive as part of your regular cancer care (I-131 and thyrotropin alfa).