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The purpose of this study was to evaluate the incidence of incidental differentiated thyroid carcinoma in thyroid operations for a benign preoperative diagnosis, to identify the risk factors involved and to risk stratify the cancer patients according to the 2015 American Thyroid Association (ATA) guidelines.The study was a retrospective review of all thyroidectomy operations performed in a single institution (January 2004 to January 2009). We excluded patients with a preoperative diagnosis of thyroid malignancy.Incidental differentiated thyroid carcinoma was diagnosed in 282/1369 patients (21%). The incidental group had a significantly higher number of males (19% vs 14%, P = 0.033) and a higher number of patients with histopathological evidence of thyroiditis (35% vs 25%, P = 0.004). There was a higher number of lymph nodes present in the incidental group but numbers did not reach statistical significance (17% vs 13%, P = 0.079). There were 270 cases in the ATA low-risk group (96%) and 12 cases in the ATA intermediate-risk group (4%). Patients with an ATA intermediate risk had a statistically higher number of capsule invasion, extrathyroidal extension and angioinvasion (P < 0.001, P < 0.001 and P < 0.001, respectively). Overall, 22% of patients with an incidental differentiated thyroid carcinoma should be considered for radioactive iodine 131I treatment. 29 of the 191 patients in American Joint Committee on Cancer stage I should be considered for radioactive iodine treatment (15%).Males and patients with thyroiditis are at a higher risk for an incidental differentiated thyroid carcinoma. One of every five of patients diagnosed with cancer will need radioactive iodine treatment, even some patients with stage I disease.