The guidelines for the management of thyroid nodules and differentiated thyroid cancer were first published by the American Thyroid Association in 1996 and subsequently updated in 2006, 2009, and 2015. In light of advances in technology and the accumulation of new scientific evidence, the guidelines were revised once again in 2025. The most notable change in the 2025 version is the exclusion of thyroid nodules, with the focus placed solely on differentiated thyroid cancer. This review aims to provide an overview of the major recommendation level changes introduced in the 2025 ATA guidelines compared with the previous version. This part specifically addresses the updates regarding the diagnosis and surgical management of differentiated thyroid cancer.