Introduction: This study aimed at comparing thyroid cancer staging when taking into account the differences between the “T” assessment” using ultrasound (US) and histopathological measurements. Material and Methods: This retrospective study included all consecutive differentiated follicular thyroid cancer (DTC) and medullary thyroid cancer (MTC) patients who underwent postoperative histopathological staging assessment at a single institution. Anaplastic thyroid carcinomas were excluded from the present study. Each malignant thyroid nodule was precisely evaluated by measuring its long axis using both US and gross specimen histopathological examination. T stage classification was attributed to each tumor as regards US (solely according to the tumor dimension) and histopathology: (1) solely according to the tumor dimension and (2) according to the tumor dimension and extrathyroidal extension features when present. Results: Retrospective comparison between US and histopathology size of the operated thyroid nodules showed a mean diminution of 7.52% of the tumor long axis. Tumors ≤10 mm at histopathological examination showed a larger decrease in size of 13% (p = 0.054, statistically significant) compared to the US measurements. Ten out of 72 (13.8%) patients showed final T downstaging in comparison to US assessment: (US) T2 to T1b in 6 patients (1 MTC) and (US) T1b to T1a in 4 patients (1 MTC). Two (2.9%) DTC patients were downstaged from stage 2 to stage 1. Conclusion: Precise thyroid tumor US measurement may differ significantly from that obtained by histopathological assessment, which may result in a different TNM staging and subsequent patient management.