American Thyroid Association (ATA) guidelines for the management of thyroid nodules and differentiated thyroid cancer (DTC) were first published in 1996 and updated in 2006, 2009, and 2015. In 2025, the ATA released a revised version that, for the first time, focuses exclusively on DTC. In our previous reviews, we summarized the updates on preoperative, diagnostic, surgical, and early postoperative management of DTC. In this third and final part, we compare the 2015 and 2025 ATA guidelines with respect to long-term follow-up, TSH suppression strategies, thyroglobulin monitoring, follow-up of low-risk patients. Diagnostic radioiodine whole-body scanning, FDG-PET/CT, dynamic risk stratification, and the management of recurrent or metastatic disease, including local therapies, radioiodine preparation and dosing, and systemic treatments were also highlighted in this part. Particular emphasis is placed on the expanded recommendations for kinase inhibitor therapy, sequencing of systemic agents, targeted therapies (NTRK, RET, ALK, BRAF), redifferentiation approaches, immunotherapy, and supportive care strategies addressing bone and brain metastases, financial toxicity and psychosocial needs. This review provides a comprehensive synthesis of these updates and discusses their implications for individualized long-term management of DTC.