Background: Malignant serous cavity effusion caused by primary thyroid cancer is extremely rare in routine clinical practice. Therefore, it is often not included in the differential diagnostic workup of patients presenting with positive effusion cytology. Methods: The clinical features were reviewed for 6 patients seen at our institution over the last 26 years for malignant effusion resulting from metastatic thyroid cancer. The cytomorphology from 4 of these cases was also reviewed. Results: All of the patients found in this study presented with malignant pleural effusion – other serous cavity effusions resulting from metastatic thyroid carcinoma were not seen. These comprised 0.25% of all patients with a known history of thyroid carcinoma and 0.67% of all malignant pleural effusions. One patient had a history of bone metastases, but all the others had no pathological evidence of distant metastatic disease prior to the pleural effusion. Conclusions: Malignant pleural effusion rarely results from a thyroid carcinoma after some latency. The diagnosis requires immunohistochemical staining as well as the pertinent clinical context.