Purpose: Toassess the clinic-pathological, diagnostic & therapy outcome differences between I-131 avid and FDG avid loco-regional recurrence in differentiated thyroid carcinoma (DTC) patients.Materials and methods: 125 patients with pathologically proved loco-regional recurrent DTC afterinitial remission with complete radio-surgical thyroid ablation were included. Clinico-pathological data, serum thyroglobulin levels, diagnostic radiology, I-131 &FDG PET/CT scanning, therapeutic approaches and follow up data were collected.Results:125patients with loco-regional recurrence were divided into: (I) I-131 avid
group:60 patients with 112 confirmed recurrent malignant lesions (8 thyroid bed, 104 nodal). (II)
FDG avid group: 65 patients with 206 confirmed recurrent malignant lesions (14 thyroid bed, 192 nodal). Out of clinicpathological data, significantly higher pathological grading was demonstrated in FDG avid group (49 patients) compared to I-131 avid group (18 patients) P<0.05. In I-131 avid group. Thesensitivity, specificity and accuracy for locoregional recurrence using I-131WBS were 77% ,100%&81% and 87.5%,30% and 77% using ultrasound (US) respectively . In FDG avid group the sensitivity, specificity and accuracy for FDGPET/CT were95.1%, 76.1% and 91.5%, for neck US were78%, 37.5% and 72.2% and for CT were 83%, 47%and 79% respectively. The added value of post-operative RAI-131 therapy results in better outcome in I-131 avid group.Conclusions:FDG avid loco-regional recurrence seemed to be linked to higher pathological grading with less favorable prognosis compared to I-131 avid loco-regional recurrence. Specificity rather than sensitivity indices tend to be significantly higher with functional compared to anatomical based techniques in both groups.