Lack of association between Hashimoto thyroiditis and breast cancer: a quantitative research synthesis

Several authors have suggested a positive association between Hashimoto thyroiditis (HT) and breast cancer (BrCa). Others have refuted these findings; hence, this subject remains controversial. We therefore reviewed the world literature on this subject accumulated over the last 50 years and performed a quantitative research synthesis, a meta-analysis variant. The incidence risk ratios and 95% confidence intervals (CI's) were calculated for each study and the combined relative risk (RR) was estimated. We found 37 relevant studies, of which only 13 were accessible to analysis. A significant association (RR=1.40; CI=1.29-1.53, P<0.022) was found for 6 of the 13 studies pertaining to 1,431 women. However, in the cumulative population of 14,226 women (from all 13 studies), we failed to demonstrate an association between the diagnoses of HT and BrCa (RR=1.07; CI=0.99-1.15; P=0.08). In conclusion, we believe that selection bias or institutional referral bias, in at least some of the "positive" studies, may have led to the spurious recognition of an association between HT and BrCa, especially as both of these conditions are highly prevalent in women between the 4th and 7th decade of life.