Description: Patients with breast cancer have a relatively high prevalence of diffuse thyroid uptake of 18 F-FDG related to thyroid autoimmunity. It is postulated that the presence of thyroid autoimmunity has prognostic implications for breast cancer. The aim of this study was to evaluate the prognostic value of incidental diffuse thyroid uptake in breast cancer patients. Methods: This was a retrospective observational cohort study in a tertiary referral hospital. We evaluated a total of 564 patients who had undergone surgery for primary breast cancer between January 2006 and December 2009. Patients were divided into 2 groups according to their diffuse thyroid uptake. The main outcome measure was disease-free survival. Results: Of the 564 patients, 108 (19.1%) showed diffuse thyroid uptake. The median follow-up period was 36.0 mo (range, 1.0–77.0 mo). Both thyroperoxidase and thyroglobulin antibody titers were higher in patients with thyroid uptake than in those without ( P 〈 0.001 for both). Of the 108 patients with thyroid uptake, 5 had a recurrence of breast cancer during the follow-up, whereas 85 without uptake had a recurrence (log-rank statics, 12.28; P 〈 0.001). The association between diffuse thyroid uptake and tumor recurrence was not significant in multivariate analysis of patients with early-stage breast cancer (hazard ratio, 0.26; 95% confidence interval, 0.06–1.10; P = 0.067). However, the association between diffuse thyroid uptake and breast cancer recurrence was statistically significant in multivariate analysis with adjustment for several prognostic variables (hazard ratio, 0.19; 95% confidence interval, 0.57–0.62; P = 0.006). Conclusion: Incidental diffuse thyroid uptake related to autoimmune thyroiditis was an independently favorable prognostic factor in advanced breast cancer. These findings support evidence that thyroid autoimmunity has a beneficial effect on the outcomes of breast cancer patients.