Central Adiposity, Obesity, Metabolic Syndrome, and the Risk of Thyroid Cancer in Adults Aged ≥75 Years: A Nationwide Korean Cohort Study

<b>Background</b>: The contribution of adiposity and metabolic syndrome (MetS) to thyroid cancer risk in late life, particularly among the elderly, is unclear. <b>Methods</b>: We conducted a nationwide cohort study of Korean adults aged ≥75 years who underwent standardized health screening. Exposures were body mass index (BMI), waist circumference (WC), and MetS defined by standard clinical criteria. The incidence of thyroid cancer was determined using administrative data. Fine-Gray sub-distribution hazard models estimated adjusted hazard ratios (HRs) with prespecified stratification by sex and age (75-84 vs. ≥85 years). <b>Results</b>: Among 1,164,707 participants (60.3% women), 2645 incident cases were identified. In the fully adjusted models, obesity (BMI ≥ 25 kg/m²) was associated with a 37% higher hazard (HR, 1.37; 95% confidence interval [CI], 1.27-1.49) and MetS with an 18% higher hazard (HR, 1.18; 95% CI, 1.09-1.28). In sex-stratified models, MetS was associated with thyroid cancer in women (HR 1.19; 95% CI, 1.08-1.31) and showed a similar direction of association in men (HR 1.16; 95% CI, 1.00-1.35), with overlapping CIs. By age, associations were evident at 75-84 years (MetS: HR, 1.18; obesity: HR, 1.36), whereas at ≥85 years, only obesity remained significant (HR, 1.90; 95% CI, 1.13-3.18). Among MetS components, high WC showed the most consistent association (HR, 1.31; 95% CI, 1.21-1.42). <b>Conclusions</b>: In adults aged ≥75 years, general obesity and, in particular, central adiposity are robustly associated with incident thyroid cancer, whereas metabolic syndrome confers a more modest and mainly age- and sex-specific additional risk.