Breastfeeding and thyroid cancer risk in women: A dose-response meta-analysis of epidemiological studies

Summary Background & aims The association between breastfeeding and thyroid cancer risk is not consistent from epidemiological studies. To better clarify the association including assessing a potential dose–response relationship, we conducted a comprehensive meta-analysis. Methods We searched PubMed (MEDLINE) up to November 2015 for prospective studies or case-control studies that evaluated the association between breastfeeding and risk of thyroid cancer. Effect estimates were pooled using a fixed-effects model. Results Nine reports (2 prospective studies, 6 case-control studies and 1 pooled analysis of 14 case-control studies) involving 2423 cases and 350,081 non-cases were identified. After pooling relevant studies, there was a significant inverse association between ever breastfeeding and risk of thyroid cancer (RR = 0.91, 95% CI 0.83–0.99), with minor heterogeneity (I 2  = 10.1%). The dose-response analysis revealed a significant linear relationship between the duration of breastfeeding and risk of thyroid cancer. The summary RR for an increment of 1 month of breastfeeding with risk of thyroid cancer was 0.983 (95% CI 0.98–0.99). When focusing on cohort studies, a more prominent linear dose–response relationship was detected, with the combined RR for every increment of 1 month of breastfeeding to be 0.965 (95% CI 0.96–0.97). Conclusions This meta-analysis suggests that breastfeeding is potentially inversely associated with thyroid cancer risk. Also longer duration of breastfeeding may further decreases thyroid cancer risk. If validated in large-scale prospective studies, our findings may have implications for impacting women's decision in breastfeeding.