Objective. To identify cytological FNA of thyroid nodules that would be unnecessary to perform according to US classification systems applied in our environment (ATA and ACR TI-RADS). Material and Methods. n=346 thyroid nodules were US and cytological evaluated in 293 patients (238 women and 55 men). Mean age without statistical significance between sexes (48.4/47.6 years old for males/women). Results. When the nodule characteristics are suspicious, both systems were similar in suggesting an FNA. But to discern which nodules not require FNA, ACR TI-RADS was significantly better than ATA classification, in terms of lower amount of Positive False and Specificity (P<0.05). Complications: 5/293 (1.7%) superficial hematomas (early complications); no late complications were observed. Why were there FNA in nodules that had FNA even when both classifications suggested not to do so? doctor´s request (43.4%) and/or patient preference (32.4%), nodules in a context of a chronic thyroiditis parenchyma (12.4%) and purely cystic nodules >35 mm, to evacuate the liquid content (11.8%). Conclusion. Nodules cytologically identified as suspicious was low. 94% of the FNA performed only required clinical follow-up. ACR TI-RADS classification induces fewer unnecessary FNA than ATA classification. Both classifications are wrong in equal proportion in suggesting no FNA suspicious nodules.