Does thyroid dysfunction increase the risk of breast cancer? A systematic review and meta-analysis.

PURPOSE: To investigate the relationship between hypothyroidism, hyperthyroidism, thyroid hormone replacement, and the risk of breast cancer.

METHODS: We searched the PubMed, Cochrane Library, EMbase, Web of Science, and China Biology Medicine (CBM) databases through June 2016 to identify researches that assessed the relationship between thyroid dysfunction and the risk of breast cancer together with the impact of thyroid hormone substitution treatment on incidence of breast cancer. Quality of evidence was assessed per outcome, using GRADE.

RESULTS: A total of 13 population-based studies including 24,808 participants were identified as eligible for this meta-analysis. A meta-analysis of 12 researches illustrated that hypothyroidism was not related to the risk for breast cancer [odds ratio (OR) = 0.83, 95% confidence interval (CI) 0.64-1.08, P = 0.162]. 10 researches illustrated that hyperthyroidism was also not related to the risk of breast cancer (OR = 1.03, 95% CI 0.83-1.30, P = 0.767). The impact of therapy was evaluated in six researches; there was no proof of a relationship between thyroid hormone substitution treatment and breast cancer with an overall OR of 0.83 (95% CI 0.57-1.21, P = 0.965).

CONCLUSIONS: Our meta-analysis illustrated that thyroid dysfunction may not be related to increased risk of breast cancer as well as the thyroid hormone substitution treatment did not reduce the incidence of breast cancer; while this study has some confounders that might weaken the results of this meta-analysis, we believe that the findings provide valuable information for stakeholders concerned with outcomes in patients with thyroid dysfunction.