Cardiac metastasis in thyroid cancer is extremely rare. Iodine-131-d whole-body scan has been used widely to detect thyroid metastasis. However, in dedifferentiated cases, iodine scan has low diagnostic value particularly for diagnosing cardiac metastasis. In the absence of (131)I uptake, (18)F-fluoro-2-deoxyglucose positron emission tomography ((18)F-FDG PET) can be used as an alternative and has a high sensitivity for thyroid metastasis, but still low sensitivity for cardiac metastasis. Therefore, meticulous attention to the pattern of uptake and comparison with patients' previous studies is critical. Additionally, cardiac magnetic resonance imaging (MRI) can provide additional and critical information.