Background <p>Patients with breast cancer can develop hypercalcemia as a metabolic complication of this disease. Bone metastasis is the most common reason responsible for this complication. Yet, hyperparathyroidism is a rare cause of hypercalcemia in these patients. We present a unique case of parathyroid adenoma-related hypercalcemia in a patient with breast cancer and its diagnostic challenges that can alter patient treatment and outcome.</p> Case presentation <p>A 50-year-old Azeris woman presented with lethargy and weakness. She had a history of recently diagnosed breast cancer and had a left mastectomy 2&#xa0;months ago. She was under chemotherapy at the time the symptoms started. Laboratory tests showed elevated serum calcium levels. Although initially, bone metastasis was considered as a cause of hypercalcemia, later investigation showed no metastasis. Elevated PTH levels along with persistent hypercalcemia after medical treatments revealed the need to do an ultrasonography of the thyroid and parathyroid glands, which showed a mass in the right parathyroid gland. After a surgical consultation, the patient underwent neck exploration. The only enlarged parathyroid gland was in the right lower, and it was removed successfully. Calcium levels decreased significantly after surgery, and the patient’s symptoms improved. She was discharged 2&#xa0;days after surgery and showed no recurrence during follow-ups. The pathology result was compatible with parathyroid adenoma.</p> Conclusion <p>This case, along with similar ones reported before, shows the challenges of diagnosing the cause of hypercalcemia in patients with breast cancer. Although hyperparathyroidism is rarely encountered, it can be managed and treated with better outcomes than bone metastasis. It highlights the need for more detailed diagnostic protocols to ensure better management.</p>