Occult thyroid carcinoma preceded by clinical manifestations and findings from extrathyroidal tumors is rare. The lack of malignant findings in the thyroid during the preoperative examination makes diagnosis difficult. We encountered a 71-year-old man with a primary ectopic thyroid carcinoma causing superior vena cava syndrome. Although no malignant findings were found in the thyroid gland, biopsy of bone metastases led to the diagnosis of thyroid cancer. HE staining of bone metastases revealed nuclear features of papillary carcinoma, and immunostaining was positive for thyroglobulin and PAX-8. The second case involved an 84-year-old man with a mediastinal tumor and suspected thyroid cancer because of high thyroglobulin levels in blood. The pathological tumor finding was papillary thyroid cancer. The last case was that of a 56-year-old woman lacking preoperative thyroid examination malignant findings, but with cervical lymph node metastasis. The thyroglobulin level of the lymph node puncture fluid was useful for preoperative diagnosis. We performed total thyroidectomy plus bilateral modified neck dissection. Pathology revealed a 1 mm papillary carcinoma in the left lobe. All of these cases were difficult to diagnose. However, we combined the results of various tests such as radiographic imaging, blood tests, and immunohistological tests to diagnose our patients.