Abstract
<div>AbstractPurpose:<p>Thyroid disease is a frequent comorbidity in women with breast cancer, and many require thyroid hormone replacement therapy (THRT). We postulated that THRT has a deleterious clinical effect mechanistically through hormonal interactions, nuclear receptor cross-talk, and upregulation of high-risk breast cancer genes.</p>Experimental Design:<p>Observational studies of patients with lymph node–negative (LN<sup>−</sup>) breast cancer (<i>n</i> = 820 and <i>n</i> = 160) were performed to test interactions between THRT and clinical, histologic, outcome, and treatment variables. Differences between the two cohorts include but are not limited to patient numbers, decades of treatment, duration of follow-up/treatment, tumor sizes, incidence, and type and dose/regimen of antihormonal and/or chemotherapeutic agents. <i>In vivo</i> and <i>vitro</i> models, <i>in silico</i> databases, and molecular methods were used to study interactions and define mechanisms underlying THRT effects.</p>Results:<p>THRT significantly and independently reduced disease-free and breast cancer–specific overall survival of only the steroid receptor (SR)-positive (as compared with SR-negative) node-negative patients in both long-term observational studies. Patients with SR<sup>+</sup> LN<sup>−</sup> breast cancer who received THRT and tamoxifen experienced the shortest survival of all treatment groups. A less potent interaction between THRT and aromatase inhibitors was noted in the second patient cohort. Using <i>in vivo</i> and <i>in vitro</i> models, TH administration enhanced estrogen and TH-associated gene expression and proliferation, nuclear colocalization of estrogen receptor and thyroid hormone receptor, and activation of genes used clinically to predict tumor aggression in SR<sup>+</sup> breast cancer, including the <i>IGF-IR</i>, <i>WNT</i>, and <i>TGFβ</i> pathways.</p>Conclusions:<p>We show clinically significant adverse interactions between THRT, estrogenic, and oncogenic signaling in patients with SR<sup>+</sup> LN<sup>−</sup> breast cancer.</p></div>
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