Cyclophosphamide and Sirolimus for the Treatment of Metastatic, RAI-refractory, Differentiated Thyroid Cancer

This trial is studying a combination of two drugs — cyclophosphamide (a mild chemotherapy) and sirolimus (an immune-modulating drug) — in people with a type of thyroid cancer that no longer responds to radioactive iodine treatment. **You may be eligible if...** - You have been diagnosed with differentiated thyroid cancer (a common type of thyroid cancer) - Your cancer cannot be cured with standard treatment, or you have declined curative treatment - Your cancer has measurable tumors and has been growing despite treatment - Your cancer no longer responds to radioactive iodine (I-131) therapy, or you are not a candidate for it - You are 18 years or older - You are able to do most normal daily activities - Your kidneys, liver, and blood counts are within acceptable ranges - You agree to use contraception if you could become pregnant **You may NOT be eligible if...** - Your cancer has not been tested with advanced genetic analysis (Next Generation Sequencing) - You received chemotherapy, targeted therapy, or radiation within the last 4 weeks - You have received an experimental treatment within the last 4 weeks - You have certain serious heart, lung, or other medical conditions Talk to your doctor to see if this trial is right for you.