The invasion of thyroid cancer out of the peritoneum indicates that there may be peripheral tissue invasion of the tumor, which belongs to the locally progressive stage and is more serious. The thyroid gland has two layers of peritoneum, the inner layer of peritoneum is the thyroid fibrous capsule formed by the intrinsic fibrous membrane of the gland, and the outer layer of peritoneum is the pre-tracheal fascia, which surrounds the thyroid gland and its blood vessels. Thyroid cancer invading out of the peritoneum indicates that the cancer foci have broken through the thyroid tissue and are not confined to the thyroid gland, and there may be extra-thyroidal infiltration or even infiltration of the trachea and the recurrent laryngeal nerve, and the subsequent regional lymph nodes and distant metastasis are more likely to occur. At this time, the staging is at least stage II, which belongs to the middle-late stage of thyroid cancer and is more serious. However, the prognosis of thyroid cancer is also related to the type of pathology, lymph nodes and distant metastasis, etc. It is recommended that patients consult with specialists in a timely manner, combine with other examination results for comprehensive assessment, and follow the doctor’s instructions for further examination and treatment.