No Causal Link Found Between Obesity and Thyroid Cancer Risk

Obese mans thyroid
Illustration of an obese man’s thyroid gland.
In Mendelian randomization analyses, obesity was not causatively linked to either benign nodular thyroid disease or thyroid cancer.

Obesity is not causatively linked to either benign nodular thyroid disease or thyroid cancer, according to study results published in The Journal of Clinical Endocrinology & Metabolism.

Previous studies have reported an association between adiposity and thyroid cancer and there are data indicating that obesity is associated with more aggressive features of thyroid malignancies. In the current study, researchers explored the association between obesity and other adiposity-related factors and the risk for benign nodular thyroid disease and thyroid cancer. Mendelian randomization was performed to assess the effects of polymorphisms associated with obesity.

The researchers used data from the UK Biobank on unrelated individuals of European ancestry and identified 1812 (82% women; mean age, 58.92 years) with benign nodular thyroid disease and 425 (74% women; mean age, 57.72 years) with differentiated thyroid carcinoma. Separate control groups were used for the 2 cohorts so that control individuals had no history of nodular thyroid disease (n=377,896) and no history of any cancer (n=310,176), respectively.

Observational findings revealed an association between higher adiposity and an increased risk for benign nodular thyroid disease, as each standard deviation increase (4.8 kg/m2) in body mass index (BMI) was associated with a 15% increased risk for benign nodular thyroid disease (odds ratio [OR], 1.15; 95% CI, 1.08-1.22). In a similar fashion, a higher waist to hip ratio was associated with benign nodular thyroid disease (OR, 1.16; 95% CI, 1.09-1.23), but there was no association with thyroid cancer.

Mendelian randomization analysis provided no evidence for a causal role of BMI, waist to hip ratio, or waist to hip ratio adjusted for BMI in thyroid nodule or malignancy risk. Although there was also no evidence that a higher genetic liability for type 2 diabetes caused thyroid cancer, the risk for the malignancy was 45% higher in individuals in the top quartile of the genetic risk score for type 2 diabetes (OR, 1.45; 95% CI, 1.11-1.90).

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The study had several limitations, including small sample of patients with thyroid cancer, possible healthy volunteer bias in the UK Biobank database, potential recall bias as diagnosis of type 2 diabetes was based on self-report, and the low prevalence of thyroid nodules (0.5% in the study population vs 2%-6% for reported prevalence in other literature), which suggested potential detection bias with many undiagnosed nodules.

“[W]e were unable to find a causative link between obesity and either benign nodular thyroid disease or thyroid cancer. This suggests the possibility of other factors confounding the reported observational associations in the case of obesity,” concluded the researchers.

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Fussey JM, Beaumont RN, Wood AR, Vaidya B, Smith J, Tyrrell J. Does obesity cause thyroid cancer? A Mendelian randomization study [published online May 11, 2020]. J Clin Endocrinol Metab. doi:10.1210/clinem/dgaa250