Metastatic Thyroid Cancer

This study will be a non-randomized pilot trial using Cyclophosphamide and Sirolimus for the treatment of metastatic differentiated thyroid cancer. Patients will be treated with Sirolimus 4 mg, PO, days 1-28 as well as Cyclophosphamide 100 mg, PO, days 1-5 and 15-19. Cycle length will be 28 days. Patients will be monitored closely for toxicity and undergo imaging to evaluate efficacy once every 2 cycles.

This research is being done to determine the efficacy of selpercatinib to restore radioactive iodine (I-131 NaI) uptake and allow for I-131 treatment in people with RET fusion-positive radioiodine-refractory thyroid cancer. This research study involves the study drug selpercatinib in combination with standard of care treatments, I-131 and thyrotropin alfa (rhTSH).