Introduction. Numerous clinical trials have proven the connection between two glandular organs, in this case, the breast and the thyroid gland. The occurrence of breast cancer (BC) is increased in patients with autoimmune thyroid disease (Hashimoto's thyroiditis and Graves' disease). Patients with Graves' disease have a significantly smaller number of described cases of BC than those with diagnosed Hashimoto's thyroiditis. Case report. A 57-year-old female patient came to the emergency center with difficulty breathing. During the examination, ophthalmopathy, weakened breath sound and mastitis of both breasts were found. Hormonal analysis showed the following values: TSH 0.00 (0.3-5.5 mlU/L), FT4 32.90 (11.5-23 pmol/L), TSHRAt 19 (0.0-1.1 U/L), TPOAt 234 (0.0-12 IU/ml), TgAt > 2000 (0.0-30.0 IU/ml). A diagnosis of Graves' disease was established and therapy with thyrosuppressant was started immediately. A multi-detector computed tomography (MDCT) showed a left breast tumor with metastases in the supraclavicular and axillary lymph nodes, infiltration of the tumor into the skin and subcutaneous tissue, as well as metastases in the bones. A biopsy of the breast tumor was performed, and PH findings indicated poorly differentiated ductal carcinoma of the breast, the HER-2+ group of tumors. Conclusion. The early detection of thyroid disease would not lead to the development of a malignant process, and that is why doctors in their clinical work must recognize the first signs of thyroid disease in their patients and immediately start with therapy to reduce the potential risk of BC. There is a significant role in using screening tests to discover breast cancer in patients with untreated or inadequately treated hypo-and hyperthyroidism.