Parathyroid carcinoma is a rare cause of primary hyperparathyroidism. Clinical indicators of carcinoma include the severity of hyperparathyroidism and presence of a palpable neck mass. Definitive diagnosis requires surgical resection and specimen histology, or documentation of metastatic disease. We present a case of parathyroid carcinoma in a young female patient with severe hyperparathyroidism, who was also found to have subclinical hyperthyroidism in the setting of an intercurrent, solitary, functioning thyroid adenoma. A diagnostic approach to parathyroid disease, combining multimodality imaging and clinicopathologic features, is discussed.