Thyroid metastases from breast cancer: A report of two cases and a literature review

Breast cancer is the most common malignant tumor in women worldwide. Common sites of distant metastasis include bone, liver, brain and lung, while metastasis to the thyroid is relatively rare. The present study aimed to investigate the clinical characteristics, diagnostic challenges and treatment strategies of breast cancer metastasis to the thyroid through two case reports and a structured literature review. Clinical data from 2 patients with breast cancer who developed thyroid metastasis after surgery were retrospectively analyzed. A structured literature search was conducted in PubMed for reports published between January 2010 and December 2025. In total, 44 cases of breast cancer metastasis to the thyroid, including the 2 cases reported in the present study, were reviewed for pathological characteristics, immunohistochemical profiles, ultrasonographic findings and treatment approaches. For variables with missing information, analyses were performed using only cases with available data as the denominator. Both patients experienced recurrence after initial treatment for breast cancer. Case 1 presented with diffuse thyroid involvement and cervical lymphadenopathy, whereas Case 2 presented with a focal thyroid nodule in the setting of widespread metastatic disease. Fine‑needle aspiration, histopathological examination and immunohistochemical analysis supported the diagnosis of metastatic breast carcinoma. Invasive ductal carcinoma was the most frequently reported histological type. Among cases with available data, estrogen positivity was observed in 23 of 32 cases (71.9%), and progesterone receptor positivity was observed in 21 of 32 cases (65.6%). Ultrasonographic information was available for 39 cases: 23 cases (59.0%) showed a focal nodular pattern, and 16 cases (41.0%) showed diffuse parenchymal involvement. Microcalcifications were reported in 12 of the 16 diffuse cases (75.0%). Thyroid metastasis from breast cancer is rare and may mimic primary thyroid malignancy. Accurate diagnosis requires integration of the patient's oncological history, imaging findings, cytological or histopathological examination, and immunohistochemical profile. Increased clinical awareness and targeted thyroid evaluation may facilitate timely diagnosis in patients with thyroid‑related symptoms, palpable cervical abnormalities, suspicious imaging findings, cervical lymphadenopathy or established metastatic disease.