Thyroid adenoma is the most common benign tumor of the thyroid gland and is classified morphologically into follicular adenoma and papillary cystic adenoma. Follicular adenoma is common and surrounded by an intact envelope, while papillary cystic adenoma is rare but often not easily distinguished from papillary adenocarcinoma and requires pathological confirmation. The adenoma is slow growing, slightly hard, smooth and painless, and can move up and down with the swallowing motion. When the blood vessels of papillary cystic adenoma rupture and intracapsular hemorrhage occurs, the adenoma may rapidly increase in size and become swollen in a short period of time. Histologically, adenomas have an intact envelope surrounded by normal tissue and are clearly demarcated, whereas single nodules in nodular goiters have an incomplete envelope. Treatment: 20% of thyroid adenomas may develop hyperthyroidism and 10% may become malignant, i.e., transform into thyroid cancer. Therefore, once diagnosed, surgical resection is recommended, such as thyroid adenoma removal or major thyroidectomy on the affected side, and the resected specimen must be sent for pathological examination and frozen section to determine whether there is malignancy. If malignancy occurs, further treatment is required.