The cause of the association between breast cancer (BC) and thyroid autoimmunity is still unknown. Na+/I- symporter (NIS) is highly expressed in BC cells, and previous studies demonstrated that iodine content in BC is lower than in remote normal breast tissue, suggesting a disorder of iodide uptake in BC. In this study, we evaluated the presence of putative serum autoantibodies able to block the function of NIS in BC patients with thyroid autoimmunity. IgGs were obtained from: a) 11 patients with BC and high antithyroglobulin (TgAb) and antithyroperoxidase (TPOAb) autoantibodies serum concentration; b) 34 patients with Hashimoto's thyroiditis (HT) (1 was euthyroid, 4 had subclinical hypothyroidism and 29 were overtly hypothyroid); c) 15 control subjects. The biological activity of NIS was studied using a chinese hamster ovary (CHO) cell line stably expressing NIS (NIS-CHO). The course of iodide accumulation in NIS-CHO was studied after addition of Na125 I in culture medium. The accumulation of iodide linearly increased between 2 and 10 min, reaching a plateau at 45 min. The preincubation of NIS-CHO with IgGs purified from sera of BC with the highest levels of TPOAb and TgAb caused an inhibition of iodine uptake of no more than 5%. Similar results were obtained using IgGs purified from patients with HT and control subjects. Our data showed no interference of autoantibodies on iodine uptake in patients with BC and thyroid autoimmunity and the very low percentage of inhibition of iodine uptake cannot explain the lower content of iodine in BC tissue.