Parathyroid adenoma in third pharyngeal pouch cyst as a rare case of primary hyperparathyroidism.

The primitive thymus and inferior parathyroid derive from the third branchial cleft. During embryonic development, these structures descend, reaching their final localisation. Third branchial cleft anomalies present usually as a fistula, abscess or cyst. However, there are no reports on parathyroid adenomas in the literature other than as a morphological possibility. We describe the case of a 47-year-old man, who had been diagnosed with arterial hypertension and who presented with a cervical mass at the edge of the lower third of the sternocleidomastoid muscle. On ultrasonography, the mass had a cystic walled appearance. Laboratory analysis only revealed an intact parathyroid hormone level of 140.5 pg/ml. Sestamibi imaging showed a probable parathyroid adenoma in the anterior mediastinum. During surgery, a tract running from beyond the superior thyroid pedicle to the superior mediastinum was dissected and removed. In the inferior end of the tract, a brown mass was visible. Pathological examination revealed a thymus cyst surrounding a parathyroid adenoma. The primal alteration was the lack of division between the thymus and inferior parathyroid gland, and the prompt prevention of their development. In the case of our patient, a parathyroid adenoma had grown by chance.