The patient was a 74-year-old female. Edema appeared in the upper left limb 6 weeks prior to hospitalization. A computed tomography (CT) scan revealed a thrombus in the left subclavian vein with adjacent lymphadenopathy. Therefore, anticoagulant therapy was initiated. The CT scan also showed the presence of ascites and wall thickening of the greater curvature of the gastric corpus, which prompted us to perform upper gastrointestinal endoscopy. The tumor found in the greater curvature of the middle body in the stomach was biopsied and diagnosed as adenocarcinoma (signet-ring-cell carcinoma/poorly differentiated adenocarcinoma; Fig. 1). On 2-deoxy-2-(18F) fluoro-d-glucose positron emission tomography/computed tomography imaging (PET/CT), accumulation was detected in the entire stomach; in the portal and posterior area of the right hepatic lobe; in the cervical, left supraclavicular, left axillary, left parasternal, and superior mediastinal lymph nodes (which suggested extensive lymph node metastases); and interestingly and unexpectedly, in the thyroid, where 18F-FDG was diffusely taken up in both lobes, for some unknown reason (Fig. 2a, b). The patient was diagnosed with multiple metastases of gastric signet-ring-cell carcinoma/poorly differentiated adenocarcinoma, and was subsequently admitted to our hospital for chemotherapy. Serum concentrations of carcinoembryonic antigen (CEA), CA19-9, and CA125 were elevated (9.7 ng/mL, 1825.0, and 122.5 U/mL, respectively), consistent with advanced gastric carcinoma. Thyroid gland function was within the standard range. Serum thyroglobulin was supranormal (893 ng/mL). Anti-thyroglobulin and anti-TPO antibodies were negative (Table 1).   
On day 8 of hospitalization, the patient complained of bilateral neck swelling with pain on the left side. A CT scan was ordered, and it showed edema of the chin, neck, and anterior chest and increased density of adipose tissue. As for the thyroid, both the lobes were diffusely enlarged and exhibited low density on CT (Fig. 2c). Because PET revealed that a significant accumulation in the whole thyroid and antithyroid autoantibodies was negative, we suspected diffuse infiltration of gastric carcinoma cells into the thyroid. Therefore, ultrasound examination and aspiration cytology were performed.
On ultrasonography, diffuse enlargement of the thyroid gland was seen (right lobe, 51 x 25 x 26 mm; left lobe, 49 x 51 x 20 mm; thickness of the isthmus, 10 mm), and it appeared internally heterogeneous. Nodular lesions were not detected, and hypoechoic reticular lines were observed in some places. On color Doppler imaging, the blood-flow signal was low (Fig. 3: imaging by Aplio-XG; TOSHIBA Medical Systems Corporation). The fine-needle aspiration cytology sample obtained from the right lobe revealed discohesive cells with severe nuclear atypia. Notably, there were cells with large cytoplasmic mucin vacuoles and cells with a high nuclear-to-cytoplasmic ratio. These findings indicated that the thyroid lesion was categorized as "malignant" (metastatic carcinoma) according to the Bethesda system for reporting thyroid cytopathology (TBSRTC), and were consistent with the metastasis of the gastric signet-ring-cell carcinoma/poorly differentiated adenocarcinoma to the thyroid gland (Fig. 4). In addition, skin biopsy from the swollen anterior chest revealed insular tumor cells in the dermis, small blood vessels, and lymphatics. The vascular and lymphatic microembolizations caused by tumor cells established the diagnosis of cutaneous metastasis. Subsequently, the patient's respiratory tract edema worsened, as confirmed by laryngoscopy, and glucocorticoids were administered. Chemotherapy with paclitaxel was done only once, because severe cytopenia occurred and her performance status became Eastern Cooperative Oncology Group (ECOG) class 4. Despite our best supportive care, the patient died a month after hospitalization. An autopsy was not performed.