The purpose of this study to learn more about the use of redifferentiating medications as a standard treatment for radioactive iodine/RAI-refractory thyroid cancer. This study is a registry study.

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.

All patients eligible to receive a redifferentiating agent in the adjuvant setting or in the treatment of structural metastatic disease not enrolled in a clinical trial will be approached for recruitment. All eligible patients will enroll in the study and will be followed indefinitely after enrollment.

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.