Background: Intrathyroidal parathyroid adenomas (ITPAs) are a rare entity. The aim of this study is to describe the experience of 2 endocrine surgery centers and to distinguish characteristics of intrathyroidal parathyroid adenoma and nonintrathyroidal parathyroid adenomas. Methods: We included patients who had undergone operations for primary hyperparathyroidism who had intrathyroidal parathyroid adenomas. Patients with single intrathyroidal parathyroid adenomas were also compared to age- and sex-matched controls with nonintrathyroidal parathyroid adenomas. Results: Of 4,868 patients who underwent parathyroidectomy between January 2002 and June 2011, we identified 53 (1%) patients with intrathyroidal parathyroid adenoma. Sestamibi and ultrasound scans correctly identified the adenoma in 35 (70%) and 11 (61%) cases, respectively. Single adenomas were identified in 44 (83%) patients, double adenomas in 4 (8%) patients, and hyperplasia in 5 (9%) patients. Lobectomy was performed in 17 (32%) patients; enucleation was used in 36 (68%) patients. Cure was achieved in all patients and no patients experienced a recurrence. Patients with single intrathyroidal parathyroid adenomas had significantly smaller glands than patients with nonintrathyroidal parathyroid adenomas (325 ± 47 vs 772 ± 61 mg; P <.0001); however, no significant difference was identified between the groups with regard to demographics, symptoms, preoperative laboratory values, or outcomes. Conclusion: Single intrathyroidal parathyroid adenomas are smaller than nonintrathyroidal parathyroid adenomas, but patients with intrathyroidal parathyroid adenomas present with similar laboratory values and symptoms. Recognition of this rare entity can lead to a successful surgical outcome. © 2012 Mosby, Inc. All rights reserved.