BACKGROUND: The surgical success rate of parathyroidectomy is 95%. However, in rare patients, ectopic hyperfunctioning parathyroid glands may be located in the thyroid but missed on imaging studies or during surgery. The aim of the present study was to evaluate the perioperative findings in 6 patients with intrathyroid parathyroid glands treated for hyperparathyroidism in our center over a 5-year period and to review the relevant literature. METHODS: Of the 426 patients who underwent parathyroidectomy for hyperparathyroidism between 1998 and 2003 in our center, 6 (1.4%) were found to have intrathyroid glands. Their files were reviewed for clinical features, surgical approach, intraoperative findings, parathyroid gland location, and morphologic and histopathologic characteristics. RESULTS: The group included 2 men and 4 women of mean age 66 years. The diagnosis in all patients was based on an increase in serum calcium concentration (10.1-13 mg/100 mL) and parathyroid hormone level (102-320 pg/mL). The clinical symptoms were kidney stones, gastrointestinal complaints, hypertension, and osteopenia. Localization studies included ultrasonography in all patients and sestamibi in 4. When the parathyroid glands were not identified, bilateral neck and mediastinum were explored. Adenoma was diagnosed histologically in 4 patients; 2 had a double adenoma. CONCLUSION: Intrathyroid parathyroid gland is a rare condition in patients with hyperparathyroidism. In these patients, imaging may miss the pathologic gland. Despite its rarity, the possibility of an intrathyroid parathyroid should be kept in mind, and, when meticulous bilateral exploration of the neck fails to identity the hyperfunctioning gland, the surgeon should consider hemithyroidectomy.