Background <p>Collision tumors are rare but well-documented phenomena, where two distinct tumors of different origins are found together. We present a unique case of a parathyroid adenoma revealing a subclinical widespread breast carcinoma.</p> Case presentation <p>A 67-year-old Tunisian woman with a history of triple-negative breast carcinoma was diagnosed with hypercalcemia and elevated parathyroid hormone levels. Imaging studies revealed an ectopic mediastinal parathyroid adenoma and a jugulo-carotid mass. Microscopic exam showed a well-circumscribed parathyroid adenoma, surrounded by a thin fibrous capsule, containing multifocal deposits of large, atypical cells negative for parathyroid markers (synaptophysin and chromogranin) and positive for cytokeratin 7. The jugulo-carotid mass was an ectopic thyroid adenoma that was also infiltrated, confirming a subclinical spread of breast carcinoma.</p> Conclusion <p>Parathyroid metastases from breast carcinoma are rare, although more frequent than primary malignancies of the parathyroid gland. This case highlights the need to consider metastatic involvement in cases of parathyroid adenoma, particularly in patients with a history of malignancy. Further research is warranted to elucidate the mechanisms underlying tumor-to-tumor metastases and to assess its impact on clinical management and prognosis.</p>