RecruitingPhase 2Drug trialCyclophosphamide and Sirolimus for the Treatment of Metastatic, RAI-refractory, Differentiated Thyroid CancerThis 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.