Programmed death 1 or (PD-1) PD-1/PD-L1 expression will be measured and reported for patients undergoing therapy with pembrolizumab. This will help determine if there PD-1 or PD-L1 are predictive biomarkers for anti-PD-1 therapy. It will also add to the data regarding their frequency in aggressive thyroid cancer. Where available, genomic profiling data will be analyzed to determine if there is a correlation between response and mutational status.