A Study to Collect Information About the Use of Redifferentiating Medications as a Standard Treatment for Thyroid Cancer

Inclusion Criteria: * Patients ≥ 18 years old at time of consent Inclusion Criteria: * Patients diagnosed with RAI refractory thyroid cancer or unlikely to respond to RAI. * Patients who will receive redifferentiation agents prior to planned diagnostic and/or therapeutic use of radioactive iodine as part of routine clinical care. * Patients who have never received redifferentiation agents outside of a clinical trial or have received redifferentiation agents in the context of a clinical trial will be enrolled after the clinical team has decided to proceed with a redifferentiation agent as part of routine clinical care prior to diagnostic and/or therapeutic radioactive iodine administration. * Patients previously treated with redifferentiation agents as part of clinical care prior to diagnostic and/or therapeutic radioactive iodine administration who are willing to provide retrospectively collected data surrounding the time of the previous treatment and will provide prospective data from the time of consent. Exclusion Criteria: * RAI refractory thyroid cancer patients that will receive redifferentiation agents prior to diagnostic and/or therapeutic use of radioactive iodine as part of a prospective clinical trial.