Background: Evidence on the association between obesity and thyroid cancer in very old adults remains limited. We examined the associations of body mass index (BMI), waist circumference, and metabolic syndrome (MetS) with thyroid cancer risk in Korean adults aged 75 years or older.

Methods: This nationwide cohort included 1,164,707 adults aged ≥75 years who underwent a national health screening examination. Obesity was defined as BMI ≥25 kg/m², and MetS was defined using standard clinical criteria. Incident thyroid cancer was identified from national health insurance claims. Adjusted hazard ratios (HRs) were estimated using Cox proportional hazards models. Analyses were also performed according to sex and age group (75–84 and ≥85 years).

Results: A total of 2,645 thyroid cancers occurred during follow-up. Obesity was associated with an increased risk of thyroid cancer (HR 1.37, 95% CI 1.27–1.49). MetS was also associated with increased risk, although the magnitude was smaller (HR 1.18, 95% CI 1.09–1.28). The HR for MetS was 1.19 (95% CI 1.08–1.31) in women and 1.16 (95% CI 1.00–1.35) in men. In participants aged 75–84 years, both obesity and MetS were associated with thyroid cancer. In those aged ≥85 years, obesity remained associated with thyroid cancer (HR 1.90, 95% CI 1.13–3.18), whereas the association with MetS was not statistically significant. Among the individual MetS components, increased waist circumference showed the strongest and most consistent association (HR 1.31, 95% CI 1.21–1.42).

Conclusion: Obesity and increased waist circumference were associated with thyroid cancer risk in adults aged ≥75 years. The association with MetS was weaker and was not evident in the oldest age group.