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

This study collects information from patients with thyroid cancer that no longer responds to radioactive iodine treatment. Some patients can be given drugs called redifferentiation agents that can make the thyroid cancer cells respond to radioactive iodine again. This study observes how patients receive these drugs in routine clinical care — not as part of a new experimental trial — and collects data on how well they work. **You may be eligible if...** - You are 18 years or older - You have been diagnosed with thyroid cancer that doesn't respond to radioactive iodine - Your doctor has decided (as part of your regular care) to give you a redifferentiation drug before radioactive iodine treatment - You are willing to have your clinical data collected **You may NOT be eligible if...** - You are receiving redifferentiation agents as part of a different prospective (forward-looking) clinical trial Talk to your doctor to see if this trial is right for you.