RESUMO
INTRODUCTION: Parathyroid carcinoma (PC) is a rare neoplasm with a high rate of recurrence and an indolent course. It is frequently functional, causing nearly 1% of the cases of primary hyperparathyroidism (HPT), and in some cases, it may be complicated by brown tumors, mimicking bone metastases. Synchronous parathyroid and papillary thyroid carcinomas are rare. CASE REPORT: We present a patient with HPT due to PC, misdiagnosed at first evaluation, which exhibited multiple hypermetabolic lytic lesions in the skeleton, suggesting bone metastases. Their regression after PTH reduction suggested the diagnosis of brown tumors due to severe HPT. Given the persistence of HPT, the patient underwent a number of neck surgeries, and a papillary thyroid microcarcinoma with a nodal metastasis was diagnosed. A genetic test discovered a previously unreported mutation of the CDC73 (HRPT2) gene, codifying for parafibromin and resulting in a premature stop codon (c.580A>Tp.Arg194). Because of the persistence of HPT, cinacalcet therapy was started in order to control hypercalcemia. CONCLUSION: This is a very unusual patient with a newly discovered variant of the CDC73 gene and a phenotype characterized by recurrent PC, brown tumors, and N1a metastasized thyroid carcinoma. The present case confirms that PC may not exhibit clear malignant properties at first assessment, contributing to inadequate initial surgical treatment. Although infrequently, PC can be associated with papillary thyroid cancer. The diagnosis of brown tumor should be considered in patients with severe HPT and multiple destructive bone lesions mimicking metastases on PET/CT imaging.