Abstract Objective Assessment of treatment outcome in current de‐escalation for differentiated thyroid cancer (DTC) according to the 2015 Dutch thyroid cancer guidelines (NL‐15) and American Thyroid Association guidelines (ATA‐15). Design Retrospectively, the recommendations of the NL‐15 and ATA‐15 guidelines were evaluated to estimate potentially adequate, under‐ and overtreatment of DTC in patients treated in the University Medical Center Groningen between 2007 and 2017. Patients A total of 240 patients with a cT1‐T3aN0‐1aM0 DTC fulfilled the inclusion criteria. Measurements After actual treatment was given, patients were again categorized according to both guidelines into low, intermediate, or high‐risk based on tumour status. Next, they were categorized into a congruent low‐risk ( n = 60), congruent high‐risk ( n = 73), or incongruent risk group ( n = 107). Follow‐up data were used to estimate the proportion of potentially adequate, under‐, and overtreatment according to both guidelines. Results Comparing treatment recommended by NL‐15 and ATA‐15 showed significantly more over‐ and adequate treatment when following NL‐15 recommendations, and more undertreatment following ATA‐15 (all: p < .001). Subanalysis of the congruent low‐risk group showed overtreatment in 64% when following NL‐15 guidelines ( p < .001). No treatment differences were found in the congruent high‐risk group. Undertreatment was most often seen in the incongruent risk group when following ATA‐15 ( p < .001). Conclusions Low‐risk patients were treated too aggressively when following NL‐15 recommendations, where the less aggressive ATA‐15 approach seemed more adequate. Treatment of intermediate risk DTC patients varies greatly, with a relative higher rate of undertreatment according to the recommendations of the ATA‐15, advocating further refining of the risk classification in this patient group.