Obesity Is Associated with Larger Thyroid Nodules but Not with Malignant Cytology

Background: Obesity has been proposed as a risk factor for differentiated thyroid carcinoma (DTC), though findings in the literature remain conflicting. While some studies suggest an association between elevated body mass index (BMI) and thyroid malignancy, others attribute this link to diagnostic bias. The Calabria region in Southern Italy, historically affected by iodine deficiency and endemic goiter, offers a valuable population for investigating this relationship. Objective: This study aimed to evaluate the association between obesity and clinical, sonographic, and cytological characteristics of thyroid nodules in a Calabrian cohort undergoing fine-needle aspiration biopsy (FNAB). Methods: This retrospective observational study included 1192 patients evaluated at a single endocrine referral center between 2015 and 2024. Patients were stratified by BMI (<30 vs. ≥30 kg/m2). Demographic, biochemical, ultrasound, and cytological data were collected and analyzed. Cytological results were classified according to the SIAPEC 2014 system. Results: Obese patients had significantly larger thyroid nodules in terms of anteroposterior and transverse diameters, as well as overall volume (p < 0.05). However, the distribution of high-risk cytological categories (TIR 3B, TIR 4, and TIR 5) did not differ significantly between obese and non-obese patients (9.4% in both groups). Multivariate analysis confirmed that BMI was not an independent predictor of malignancy risk (OR 0.988; p = 0.723), whereas younger age was inversely associated with malignancy. Conclusions: Obesity appears to influence thyroid nodule size but does not constitute an independent risk factor for cytological malignancy. BMI should not influence indications for FNAB or subsequent treatment decisions. Thyroid nodule management should instead rely on ultrasound risk stratification and cytological findings. Special attention should be given to younger patients as they may carry a higher malignancy risk.