Objective.-To describe the phenomenon of parathyroid gland involvement by thyroid carcinoma and to explore its clinicopathologic significance. Design.-We reviewed the clinicopathologic features and described the histopathologic characteristics of 20 cases of parathyroid involvement found in a series of 911 papillary thyroid carcinomas. Results.-Parathyroid involvement occurred in 3 patterns: (1) direct invasion from the main tumor with an infiltrative growth, (2) invasion of the parathyroid gland with an expansive growth and formation of a pseudocapsule between the carcinoma and the parathyroid gland, and (3) metastatic deposition within the parathyroid gland itself. Among the 20 cases, 6 (30%) had primary tumors measuring 1 cm or less, and 14 (70%) had primary tumors measuring more than 1 cm. The cases with smaller primary tumors had a comparatively lower rate of nodal metastasis (50% vs 100%, P = .02). Furthermore, none of the cases with smaller primary tumors had invasion of extrathyroid structures other than parathyroid gland, but such invasion was common among the cases with larger primary tumors (0% vs 71%, P = -.01). Conclusion.-To the best of our knowledge, this is the first report showing that parathyroid involvement occurs in papillary thyroid carcinomas even in the early stage of the disease.
