Thyroid disease is associated with an increased risk of breast cancer: a systematic review and meta-analysis

Background This study investigated the relationship between thyroid diseases and the risk of breast cancer (BC). Clarifying this issue can help medical staff perform of early prevention, diagnosis and treatment for breast cancer patients. Methods The meta-analysis combined data from cohort studies and case-control to obtain a comprehensive result of the relationship between thyroid diseases and risk of BC. We comprehensively searched PubMed, EMbase, Web of Science, and the Cochrane Library. The search period was from the establishment of the databases to August 2020. Literature was collected and screened individually by two reviewers. There was English language restriction on the search and unpublished literature was excluded. The Newcastle-Ottawa Scale (NOS) was used to evaluate the quality of the selected studies prior to data extraction. The data collected included country, author, year of publication, research type, and number of cases. In cases where the data and study heterogeneity permitted, meta-analyses were performed, and odd ratios (ORs) with corresponding 95% confidence intervals (CIs) were calculated. Data were analyzed using the STATA 15.1 software. Results A total of 21 articles were included in this study. Hyperthyroidism, thyroid cancer, thyroglobulin antibody (TGAb) levels, and thyroid microsomal antibody (TPOAb) levels were all significantly associated with an increased risk of BC, while hypothyroidism was associated with a reduced risk of BC. Conclusions This study demonstrated that hyperthyroidism, autoimmune thyroiditis (AITD), and thyroid cancer are significantly associated with an increased risk of BC, while hypothyroidism is associated with a reduced risk of BC.