Prepregnancy Body Mass Index and Risk of Differentiated Thyroid Cancer: A Prospective Cohort Study of More than 440,000 Danish Women.

High body mass index (BMI) has previously been associated with increased risk of differentiated thyroid cancer (DTC); however, only few studies have investigated the association with BMI in a large cohort assessed at a young age and with sufficient data on confounding factors. We assessed the association between excess body weight and the risk of DTC and papillary thyroid cancer (PTC) in a large cohort of young Danish women with substantial confounder control. We included all parous Danish women registered with a prepregnancy BMI ≥18.5 kg/m during 2004-2016 in the Danish Medical Birth Registry in the study population. Cox proportional hazards regression models were used to estimate the hazard ratios (HRs) with confidence intervals (CIs) of DTC according to BMI. In subanalyses, we investigated PTC as a separate group. Analyses were adjusted for calendar time, education, smoking status, benign thyroid disease (BTD), type II diabetes, parity, and oral contraceptive use. In addition, we examined the association with increasing BMI stratified for previous BTD. A total of 443,403 women were included in the study population, and the median age at baseline was 30.0 years. Altogether, 463 women were diagnosed with DTC during follow-up. Excess body weight was associated with a higher rate of DTC (overweight, BMI 25-29.9 kg/m: HR = 1.54; CI 1.25-1.90. Obese, BMI ≥30 kg/m: HR = 1.32; CI 1.00-1.75) compared with normal weight. Results were similar in PTC. In addition, we found an increased rate of DTC with increasing BMI, when investigating BMI as a continuous variable per 5 kg/m increase (HR = 1.17; CI 1.07-1.27). The results were similar in women without previous BTD. Our study confirms that excess body weight is associated with an increased incidence of DTC and PTC in women.