Larotrectinib to Enhance RAI Avidity in Differentiated Thyroid Cancer

This study is testing whether larotrectinib — a targeted drug that blocks a specific genetic mutation (TRK fusion) — can make thyroid cancer cells more likely to absorb radioactive iodine (RAI) treatment. Some differentiated thyroid cancers stop responding to radioactive iodine, and this drug may help restore that sensitivity. **You may be eligible if:** - You are at least 1 year old - You have been diagnosed with differentiated thyroid cancer and have had your thyroid surgically removed, with appropriate local treatment - You have lung metastases visible on CT scan (either multiple small nodules or enlarging nodules) - Your cancer has a TRK gene fusion mutation - Your prior radioactive iodine scan showed low or absent iodine uptake in the cancer **You may NOT be eligible if:** - Your cancer does not have a TRK gene fusion - Your cancer absorbs radioactive iodine normally (no need to enhance uptake) - Your general health or organ function does not support participation Talk to your doctor to see if this trial is right for you.