BackgroundParathyroid carcinoma is a rare disease with a frequency of 0.005% of all malignancies [1, 2]. Various aspects of its pathogenesis, diagnosis, and treatment remain poorly understood. Furthermore, cases with secondary hyperparathyroidism are fewer. In this case report, we describe a case of left parathyroid carcinoma with secondary hyperparathyroidism.Case presentationThe patient was a 54-year-old woman who had been on hemodialysis since the age of 40 years. At 53 years of age, her calcium levels were high, and she was diagnosed with drug-resistant secondary hyperparathyroidism and was referred to our hospital for surgical treatment. Blood tests revealed calcium levels of 11.4 mg/dL and intact parathyroid hormone (PTH) levels of 1007 pg/mL. Neck ultrasonography revealed a 22-mm large round hypoechoic mass, partially indistinct margins, and D/W ratio > 1 at the left thyroid lobe. Computed tomography scans revealed a 20-mm nodule at the left thyroid lobe. No enlarged lymph nodes or distant metastases were noted.99mTc-hexakis-2-methoxyisobutylisonitrile scintigraphy revealed an accumulation at the superior pole of the left thyroid lobe. Laryngeal endoscopy revealed paralysis of the left vocal cord, signifying recurrent nerve palsy due to parathyroid carcinoma. Based on these results, a diagnosis of secondary hyperparathyroidism and suspected left parathyroid carcinoma was made, and the patient underwent surgery. Pathology results revealed hyperplasia in the right upper and lower parathyroid glands. The left upper parathyroid gland showed capsular and venous invasion, and the diagnosis was left parathyroid carcinoma. At 4 months post-surgery, calcium levels improved to 8.7 mg/dL and intact PTH levels to 20 pg/mL, with no signs of recurrence.ConclusionsWe report a case of left parathyroid carcinoma associated with secondary hyperparathyroidism. Concomitant secondary hyperparathyroidism may cause mild hypercalcemia compared to parathyroid carcinoma alone due to the added modification of dialysis. Although our patient also presented with mild hypercalcemia, a D/W ratio > 1 on preoperative echocardiography and presence of recurrent nerve palsy on laryngoscopy led to the suspicion and treatment of parathyroid carcinoma preoperatively.