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Obesity's role in thyroid cancer development is still debated, as well as its association with aggressive histopathological subtypes (AHSs). To clarify the link between Body Mass Index (BMI) and AHS of differentiated thyroid carcinoma (DTC), we evaluated patients who underwent thyroidectomy for DTC from 2020 to 2022 at four European referral centres for endocrine surgery. Based on BMI, patients were classified as normal-underweight, overweight, or obese. AHSs were defined according to 2022 WHO guidelines. Among 3868 patients included, 34.5% were overweight and 19.6% obese. Histological diagnoses were: 93.6% papillary (PTC), 4.8% follicular (FTC), and 1.6% Hürthle cell (HCC) thyroid carcinoma. Obese and overweight patients with PTC had a higher rate of AHSs ( p = 0.03), bilateral, multifocal tumours ( p = 0.014, 0.049), and larger nodal metastases ( p = 0.017). In a multivariate analysis, BMI was an independent predictor of AHS of PTC, irrespective of gender ( p = 0.028). In younger patients (<55 years old) with PTC > 1 cm, BMI predicted a higher ATA risk class ( p = 0.036). Overweight and obese patients with FTC had larger tumours ( p = 0.036). No difference was found in terms of AHS of FTC and HCC based on BMI category. Overweight and obese patients with PTC appear to be at an increased risk for AHS and aggressive clinico-pathological characteristics.