588 Background: An association between breast cancer and autoimmune hypothyroidism has been suggested. While a biological role for thyroid hormone in breast tumorigenesis has been demonstrated in vitro, epidemiological evidence is lacking. Methods: A retrospective, population-based cohort study was conducted to examine the risk of postmenopausal breast cancer (BRCA) in women with or without autoimmune hypothyroidism, identified on the basis of prescriptions for levothyroxine (LT4). Administrative health records were used to capture information on all women aged 66 and older living in Ontario, Canada at baseline (April 1, 1993 to March 31, 1996) and during a 10-year follow up. Propensity scores were used to create a matched cohort of LT4 and non-LT4 users. Cox proportional hazards modeling was used to evaluate the impact of LT4 use on the 5-year incidence of BRCA and benign breast disease, and on all-cause mortality rates among women diagnosed with breast cancer during follow-up. Results: LT4 users (N=89,093) and non-LT4 users (N=89,093) were well-matched with respect to baseline sociodemographics, estrogen use, comorbidity, and health care utilization, including the likelihood of receiving mammography or a breast biopsy. The 5-year incidence of BRCA was 0.86% in LT4 users compared to 0.97% in non-LT4 users (p=0.002). Adjustment for baseline characteristics did not alter these results (hazard ratio [HR] 0.86; 95% confidence interval [CI] 0.77–0.95; p= 0.004). Among women who developed BRCA, all cause mortality was significantly lower in LT4 users than in non-users (43.9% vs. 56.1%; adjusted HR 0.82; 95% CI 0.70–0.96; p= 0.01). The incidence of benign breast disease did not vary between groups. Conclusions: Elderly women with autoimmune hypothyroidism appeared to have a protective advantage in the incidence of BRCA and in mortality following a breast cancer diagnosis. These results suggest a biological role for thyroid hormone in the development of breast cancer, with a modulating effect of treated autoimmune hypothyroidism in promoting a less aggressive disease course. Further studies exploring the effect of thyroid hormone in breast cancer development are needed and may uncover novel therapeutic targets in the management of breast cancer in the future. No significant financial relationships to disclose.
Benign Breast Disease and the Risk of Subsequent Breast Cancer in African American Women
