Primary HT and risk for breast cancer: a systematic review and meta-analysis

Objective: The association between hypothyroidism and breast cancer has been described from very early on. Breast and thyroid tissue are interconnected on a molecular level mainly through activation of thyroid hormone receptors expressed on cells of the mammary gland as well as on the plasma membrane of breast cancer cells. Despite the experimental evidence the true value of hypothyroidism as a risk factor for breast cancer remains controversial. Methods: We searched the PubMed database through February 2011 to identify studies that evaluated the association between hypothyroidism and risk for breast cancer as well as the effect of thyroid hormone replacement therapy on breast cancer incidence. Results: A meta-analysis performed in 12 studies showed that hypothyroidism was not associated with risk for breast cancer (pooled risk ratio (RR) = 1.06, 95% confidence intervals (CIs) 0.82-1.35, P = 0.672). The effect of treatment was assessed in seven studies and no evidence for an association between thyroid hormone replacement and breast cancer was observed with an overall RR of 0.99 (95% CI 0.73-1.35, P = 0.965). Conclusions: Our meta-analysis showed that hypothyroidism is not associated with increased risk for breast cancer and thyroid hormone replacement therapy does not reduce breast cancer prevalence; however, the heterogeneity of the studies analyzed precludes firm conclusions.