An 82-year-old woman was admitted to our hospital with cough and back pain. A chest radiograph showed a solitary nodular lesion in the right lower lung field. It was diagnosed by a transbronchial biopsy as lung metastasis of a papillary adenocarcinoma of the thyroid. However, her cervical CT and ultrasonography showed only a cyst in a right lobe of the thyroid, and its biopsy did not show evidence of malignancy. In addition, multiple bone metastasis and pituitary metastasis were revealed. We therefore diagnosed this case as systemic metastasis of papillary adenocarcinoma of the thyroid. She was given best supportive care and she died seven months later. Autopsy revealed two tiny lesions (3 mm and 6 mm) in the thyroid right lobe to be papillary adenocarcinoma. We report this case because occult thyroid cancer caused systemic metastasis and the chest X-ray showed lung metastasis from the thyroid cancer as a solitary nodular lesion.