Purpose <p>Differentiated thyroid cancer (DTC) is primarily evaluated using radioiodine (RAI) scanning; however, this approach is limited by the suboptimal imaging properties of ¹³¹I, the cost and limited availability of ¹²³I, and restricted access to ¹²⁴I. [¹⁸F]Tetrafluoroborate (TFB), a PET radiotracer targeting the sodium/iodide symporter, has emerged as a promising alternative. This prospective study assessed the diagnostic performance and biodistribution of TFB PET compared with ¹³¹I scintigraphy in patients with DTC.</p> Methods <p>This prospective study enrolled 58 patients with histopathologically confirmed DTC after total or near-total thyroidectomy. Forty-eight patients underwent both TFB PET/CT and ¹³¹I SPECT scintigraphy, while 10 underwent only TFB PET for biodistribution analysis. Patients were stratified into preradioiodine (TFB before ¹³¹I; <i>n</i> = 28) and postradioiodine (TFB after ¹³¹I; <i>n</i> = 20) subgroups based on recent radioiodine administration (either diagnostic or therapeutic) to evaluate the potential stunning effect of ¹³¹I on TFB uptake. Patient- and lesion-based analyses were performed, and agreement was assessed using Cohen’s kappa.</p> Results <p>In patient-based analysis, concordance between modalities showed both TFB and ¹³¹I positivity in 20 patients, TFB-only positivity in 1 patient, ¹³¹I-only positivity in 5 patients, and negativity in both modalities in 22 patients, yielding substantial overall agreement (κ = 0.75). Agreement was almost perfect in the preradioiodine group (κ = 0.84) but declined to a moderate level in the postradioiodine group (κ = 0.49). In lesion-based analysis (<i>n</i> = 56), overall detection rates were comparable (TFB, 85.7% vs. ¹³¹I, 73.2%) but significantly higher for TFB in the preradioiodine group (95.7% vs. 68.1%, <i>p</i> &lt; 0.05). Lesion detection decreased markedly in the postradioiodine group (33.3%), consistent with a stunning effect. Biodistribution analysis showed highest physiological uptake in the stomach, salivary glands, esophagus, nasopharynx and unexpected uterine uptake in Nabothian cysts in 45% of female patients.</p> Conclusion <p>TFB PET demonstrates diagnostic accuracy and lesion detection rates comparable to ¹³¹I scintigraphy in DTC. Its diagnostic performance is superior in patients without recent radioiodine administration prior to TFB imaging, whereas lesion detectability decreases after radioiodine administration, likely due to a stunning effect. Therefore, TFB PET represents a promising alternative for disease burden assessment in DTC and warrants further multicenter validation.</p>