The objective of this study is to demonstrate the clinical utility of I-124 PET/CT imaging and dosimetry in patients with thyroid cancer including 1) Evaluation of extent (volume and pattern) of remnant tissue in post total thyroidectomy setting and distinction of nodal metastases vs remnant tissue for determination of indication for RAI ablation, 2) Evaluation of response to RAI remnant ablation, 3) Evaluation for suspected occult recurrent/metastatic disease, 4) Evaluation of extent of disease in patients with known metastatic disease and 5) Evaluation of RAI avidity of recurrent/metastatic thyroid cancer and response to treatment with thyroid kinase inhibitors (TKI). Patients who underwent total thyroidectomy for thyroid cancers are studied. Patients who are newly diagnosed, as well as those who have known or suspected to have recurrent or metastatic disease are eligible. Patients receiving TKI treatment are eligible for evaluation prior to and after the treatment. The patients who are considered for TKI/MAPK treatments undergo pre and post treatment with clinically determined oncoprotein/TKR therapeutic agent(s), including multi-TKI, selective BRAF, MEK, PI3K or ERK inhibitors or combination treatments.