Intrathyroidal parathyroid carcinoma is extremely rare clinical entity with potentially multiple diagnostic pitfalls. We report a case of 40-year-old man presented with classical manifestations of primary hyperparathyroidism, severe hypercalcemia and profoundly increased serum parathyroid hormone level. Neck ultrasonography demonstrated multinodular goiter with predominant 34 mm nodule in left thyroid lobe. Additional 16 mm nodule was found beneath the left lobe. Routine percutaneous fine-needle aspiration of predominant nodule indicated follicular thyroid carcinoma, while left inferior nodule was confirmed to be of parathyroid origin. The patient underwent surgery, during which frozen sections identified medullary thyroid carcinoma with metastasis to upper mediastinal lymph node. Permanent sections of the predominant left lobe nodule revealed intrathyroidal parathyroid carcinoma surrounded with multiple microscopic metastases. Left inferior nodule was metastatic lymph node. Additional 10 mm intrathyroidal metastasis of primary parathyroid carcinoma was found within right thyroid lobe. This case indicates that fine-needle-aspiration and intraoperative biopsy are of limited value in diagnosing parathyroid carcinoma, especially if localized intrathyroidally. Oncological en-block resection is treatment of choice, implying ipsilateral lobectomy in case of thyroid invasion. This firstly described case of intrathyroidal parathyroid carcinoma causing intrathyroidal dissemination may influence future treatment strategies.