甲状腺癌和乳腺癌是全球女性中最常见的两种内分泌相关肿瘤，且其发病率仍在上升。观察性研究表明甲状腺癌和乳腺癌之间存在关联。然而，许多混杂因素使结果容易受到干扰。因此，我们进行了一项两样本孟德尔随机化 (MR) 研究，以调查甲状腺癌和乳腺癌之间的因果关系。我们从英国生物银行（13,879 例乳腺癌病例和 198,523 例对照）和乳腺癌协会联盟（ BCAC；122,977 例乳腺癌病例和 105,974 例对照），以及来自 FinnGen Biobank 的甲状腺癌数据（989 例甲状腺癌和 217,803 例对照）。然后，执行乘法随机效应逆方差加权（IVW）、体重中位数（WM）和MR Egger方法进行MR分析。总的来说，IVW使用BCAC数据集显示了乳腺癌对甲状腺癌的因果影响（比值比[ OR] = 1.17；95% 置信区间 [CI] = 1.036-1.322；P = 0.011），并且这种关系也得到了英国生物银行数据集的支持（OR = 23.899；95% CI = 2.331-245.003；P = 0.007） ，这表明乳腺癌患者更有可能被诊断出患有甲状腺癌。总体而言，反向MR分析并未显示乳腺癌与甲状腺癌之间存在因果关系。然而，IVW 使用 BCAC 数据集显示甲状腺癌与雌激素受体阴性乳腺癌存在因果关系（OR = 1.019；95% CI = 1.001-1.038；P = 0.043），这表明甲状腺癌患者更有可能罹患甲状腺癌。乳腺癌是甲状腺癌的一个可能危险因素，而甲状腺癌也是 ER 阴性乳腺癌的一个可能危险因素。未来的研究需要使用强大的遗传工具来确定乳腺癌和甲状腺癌之间的因果关系。版权所有 © 2023 Liu 和 Liang。

Thyroid and breast cancers are the two most frequent types of endocrine-related tumors among women worldwide, and their incidence is still on the rise. Observational studies have shown a relationship between thyroid and breast cancers. Nevertheless, many confounders predispose the results to interference. Accordingly, we performed a two-sample Mendelian randomization (MR) study to investigate the causal association between thyroid and breast cancers.We acquired breast cancer data from the UK Biobank (13,879 breast cancer cases and 198,523 controls) and the Breast Cancer Association Consortium (BCAC; 122,977 breast cancer cases and 105,974 controls), and thyroid cancer data from FinnGen Biobank (989 thyroid cancer and 217,803 controls). Then, the multiplicative random effects inverse variance weighting (IVW), weight median (WM), and MR Egger methods were executed for MR analysis.Overall, IVW showed a causal effect of breast cancer on thyroid cancer using the BCAC dataset (odds ratio [OR] = 1.17; 95% confidence interval [CI] = 1.036-1.322; P = 0.011), and this relationship was also supported by the UK Biobank dataset (OR = 23.899; 95% CI = 2.331-245.003; P = 0.007), which showed that breast cancer patients were more likely to be diagnosed with thyroid cancer. On the whole, the reverse MR analysis did not show a causal effect of breast cancer on thyroid cancer. However, IVW showed a causal effect of thyroid cancer on estrogen receptor -negative breast cancer using the BCAC dataset (OR = 1.019; 95% CI = 1.001-1.038; P = 0.043), which suggested that people with thyroid cancer were more likely to develop breast cancer.Breast cancer represents a possible risk factor for thyroid cancer and thyroid cancer also represents a possible risk factor for ER-negative breast cancer. Future studies using powerful genetic tools to determine the causal relationship between breast and thyroid cancers are required.Copyright © 2023 Liu and Liang.