Background: Body weight may be a modifiable risk factor predisposing to different cancers. To establish a potential impact of weight change on thyroid cancer risk, we conducted a meta-analysis to evaluate the effect of body mass index (BMI) and weight change over time as a risk of developing thyroid cancer (TC). Methods: A systematic search was performed up to February 25, 2020. Pooled relative risk (RR) were estimated using fixed and random models. Heterogeneity between articles was examined using Q-test and I2 index. Evaluation of publication bias was conducted with Egger's regression test. Results: A total of 31 studies including 24,489,477 cohorts were eligible. Pooled analysis revealed that normal and underweight cohorts were associated with a decreased risk of TC (RR = 0.68, 95%CI = 0.65–0.71, p < 0.001) and (RR = 0.92, 95%CI = 0.91–0.93, p < 0.001), respectively. In contrast, overweight and obese cohorts were more likely to develop TC (RR = 1.26, 95%CI = 1.24–1.28, p < 0.001 and RR = 1.50, 95%CI = 1.45–1.55, p < 0.001, respectively). Obesity was associated with higher risk of developing TC among women (RR = 1.29, 95%CI = 1.14–1.46, p < 0.001), but not men (RR = 1.25, 95%CI = 0.97–1.62, p = 0.08). Furthermore, weight gain increased the risk of developing TC (RR = 1.18, 95%CI = 1.14–1.22, p < 0.001), while weight loss decreased the risk (RR = 0.89, 95%CI = 0.85–0.93, p < 0.001). Results showed similar trends of weight change effect in both males and females. Conclusions: Obesity is associated with higher risk of developing TC in women. However, maintaining a healthy weight is associated with reduced risk of TC in both women and men. Shifting our practice to include weight control strategies will help lead to cancer prevention.
