Background: Thyroid cancer incidence has increased in many parts of the world since 1980s, as has obesity prevalence. Evidence suggests that people with greater body size have greater thyroid cancer risk, but it is unclear whether this association is causal or is driven by over-diagnosis of indolent cancers because overweight/obese people use health services more frequently than those of normal weight and have greater opportunity for incidental diagnosis. Assessing whether obesity is associated with higher-risk thyroid cancers might help clarify this issue.
Methods: We recruited 1013 thyroid cancer cases diagnosed between 2013 and 2016 and 1057 population controls, frequency matched by sex and age group. We used logistic regression to assess the association between body mass index (BMI) and overall thyroid cancer risk as well as by tumour BRAF mutational status as a marker of potentially higher-risk cancer.
Results: In both women and men, higher BMI was associated with greater odds of thyroid cancer overall. Having a BMI ≥30 kg/m2 was associated with increased odds of BRAF-mutated thyroid cancer (odds ratio =2.22; 95% confidence interval: 1.66-2.97 for obese vs. normal BMI); odds ratios were lower and inconsistent for BRAF-negative cancers. The odds of BRAF-mutated cancer increased by 18% and 24% for each 5 kg/m2 increase in BMI in women and men respectively.
Conclusions: Greater risk of BRAF-mutated thyroid cancers among those with high BMI suggests that the association may not merely reflect greater healthcare service use and indicates an independent relationship between excess body fatness and clinically important thyroid cancer.