Obesity, obesity-related metabolic conditions, and risk of thyroid cancer in women: results from a prospective cohort study (Sister Study)

BACKGROUND: Thyroid cancer incidence has increased worldwide. Obesity trends may play a role, but the underlying biological pathways are not well-characterized. Therefore, we examined associations of excess adiposity and obesity-related metabolic conditions with thyroid cancer incidence. METHODS: From the Sister Study, a cohort of sisters of women with breast cancer, we included 47,739 women who were cancer-free at baseline (2003–2009). Height, weight, waist and hip circumference, and blood pressure were measured at baseline and medical history was self-reported. Cox proportional hazards regression models were adjusted for age (time scale), race/ethnicity, smoking, baseline history of benign thyroid disease, and frequency of routine healthcare visits. FINDINGS: During follow-up (median = 12.5; max = 15.9 years), 259 women reported incident thyroid cancer. Body mass index (BMI) (hazard ratio [HR](per-5 kg/m)(2) = 1.25, 95% CI = 1.14–1.37), waist circumference (HR(per-5 cm increase) = 1.11, 95% CI = 1.06–1.15), and waist-to-hip ratio (HR (≥0.85-versus-<0.85) = 1.49, 95% CI = 1.14–1.94) were positively associated with thyroid cancer incidence, as were metabolic syndrome (HR = 1.67, 95% CI = 1.24–2.25), dyslipidemia (HR = 1.46, 95% CI = 1.13–1.90), borderline diabetes (HR = 2.06, 95% CI = 1.15–3.69), hypertension (HR = 1.49, 95% CI = 1.12–1.96), and polycystic ovary syndrome (PCOS, HR = 2.10, 95% CI = 1.20–3.67). These associations were attenuated with additional BMI adjustment, although dyslipidemia (HR = 1.35, 95% CI = 1.04–1.75) and PCOS (HR = 1.86, 95% CI = 1.06–3.28) remained associated with thyroid cancer incidence. Hypothyroidism was not associated with thyroid cancer. INTERPRETATION: In this cohort of sisters of women diagnosed with breast cancer, excess adiposity and several obesity-related metabolic conditions were associated with thyroid cancer incidence. These findings provide insights into potential biological mechanisms linking obesity and thyroid cancer. FUNDING: This research was supported by the Intramural Research Program of the 10.13039/100000002National Institutes of Health, 10.13039/100000054National Cancer Institute and 10.13039/100000066National Institute of Environmental Health Sciences (Z01-ES044005).