Subclinical Hypothyroidism in Patients with Newly Diagnosed Breast Cancer

become the most common malignant disease, with two million new cases and more than 600,000 deaths in 2018, making it vital to identify the associated risk factors.1,2 Many earlier studies have attempted to assess an association between thyroid disease and breast cancer, but it is difficult to distinguish a cause-and-effect relationship between the two conditions despite their high prevalence in women.3 The relation between hypothyroidism and the risk of breast cancer was first published in 1976 by Kapdi and Wolfe, who observed a higher breast cancer incidence rate among patients undergoing thyroid supplement treatment.4 Since then, four metaanalyses have studied thyroid dysfunction and the risk of breast cancer.5-8 Angelousi et al. examined 12 studies and found no association between hypothyroidism and increased risk of breast cancer, five and further suggested that thyroid hormone replacement therapy did not reduce the prevalence of breast cancer. In a later metaanalysis, Fang et al. reported no relationship between thyroid dysfunction and risk of breast cancer, while Wang et al. found a reduced risk of breast cancer due to hypothyroidism in the European population.7,8